perlop(1)


NAME

   perlop - Perl operators and precedence

DESCRIPTION

   In Perl, the operator determines what operation is performed,
   independent of the type of the operands.  For example "$x+$y" is
   always a numeric addition, and if $x or $y do not contain numbers, an
   attempt is made to convert them to numbers first.

   This is in contrast to many other dynamic languages, where the
   operation is determined by the type of the first argument.  It also
   means that Perl has two versions of some operators, one for numeric and
   one for string comparison.  For example "$x==$y" compares two numbers
   for equality, and "$xeq$y" compares two strings.

   There are a few exceptions though: "x" can be either string repetition
   or list repetition, depending on the type of the left operand, and "&",
   "|", "^" and "~" can be either string or numeric bit operations.

   Operator Precedence and Associativity
   Operator precedence and associativity work in Perl more or less like
   they do in mathematics.

   Operator precedence means some operators are evaluated before others.
   For example, in "2+4*5", the multiplication has higher precedence
   so "4*5" is evaluated first yielding "2+20==22" and not
   "6*5==30".

   Operator associativity defines what happens if a sequence of the same
   operators is used one after another: whether the evaluator will
   evaluate the left operations first, or the right first.  For example,
   in "8-4-2", subtraction is left associative so Perl evaluates the
   expression left to right.  "8-4" is evaluated first making the
   expression "4-2==2" and not "8-2==6".

   Perl operators have the following associativity and precedence, listed
   from highest precedence to lowest.  Operators borrowed from C keep the
   same precedence relationship with each other, even where C's precedence
   is slightly screwy.  (This makes learning Perl easier for C folks.)
   With very few exceptions, these all operate on scalar values only, not
   array values.

       left        terms and list operators (leftward)
       left        ->
       nonassoc    ++ --
       right       **
       right       ! ~ \ and unary + and -
       left        =~ !~
       left        * / % x
       left        + - .
       left        << >>
       nonassoc    named unary operators
       nonassoc    < > <= >= lt gt le ge
       nonassoc    == != <=> eq ne cmp ~~
       left        &
       left        | ^
       left        &&
       left        || //
       nonassoc    ..  ...
       right       ?:
       right       = += -= *= etc. goto last next redo dump
       left        , =>
       nonassoc    list operators (rightward)
       right       not
       left        and
       left        or xor

   In the following sections, these operators are covered in precedence
   order.

   Many operators can be overloaded for objects.  See overload.

   Terms and List Operators (Leftward)
   A TERM has the highest precedence in Perl.  They include variables,
   quote and quote-like operators, any expression in parentheses, and any
   function whose arguments are parenthesized.  Actually, there aren't
   really functions in this sense, just list operators and unary operators
   behaving as functions because you put parentheses around the arguments.
   These are all documented in perlfunc.

   If any list operator ("print()", etc.) or any unary operator
   ("chdir()", etc.)  is followed by a left parenthesis as the next token,
   the operator and arguments within parentheses are taken to be of
   highest precedence, just like a normal function call.

   In the absence of parentheses, the precedence of list operators such as
   "print", "sort", or "chmod" is either very high or very low depending
   on whether you are looking at the left side or the right side of the
   operator.  For example, in

       @ary = (1, 3, sort 4, 2);
       print @ary;         # prints 1324

   the commas on the right of the "sort" are evaluated before the "sort",
   but the commas on the left are evaluated after.  In other words, list
   operators tend to gobble up all arguments that follow, and then act
   like a simple TERM with regard to the preceding expression.  Be careful
   with parentheses:

       # These evaluate exit before doing the print:
       print($foo, exit);  # Obviously not what you want.
       print $foo, exit;   # Nor is this.

       # These do the print before evaluating exit:
       (print $foo), exit; # This is what you want.
       print($foo), exit;  # Or this.
       print ($foo), exit; # Or even this.

   Also note that

       print ($foo & 255) + 1, "\n";

   probably doesn't do what you expect at first glance.  The parentheses
   enclose the argument list for "print" which is evaluated (printing the
   result of "$foo&255").  Then one is added to the return value of
   "print" (usually 1).  The result is something like this:

       1 + 1, "\n";    # Obviously not what you meant.

   To do what you meant properly, you must write:

       print(($foo & 255) + 1, "\n");

   See "Named Unary Operators" for more discussion of this.

   Also parsed as terms are the "do{}" and "eval{}" constructs, as well
   as subroutine and method calls, and the anonymous constructors "[]" and
   "{}".

   See also "Quote and Quote-like Operators" toward the end of this
   section, as well as "I/O Operators".

   The Arrow Operator
   ""->"" is an infix dereference operator, just as it is in C and C++.
   If the right side is either a "[...]", "{...}", or a "(...)" subscript,
   then the left side must be either a hard or symbolic reference to an
   array, a hash, or a subroutine respectively.  (Or technically speaking,
   a location capable of holding a hard reference, if it's an array or
   hash reference being used for assignment.)  See perlreftut and perlref.

   Otherwise, the right side is a method name or a simple scalar variable
   containing either the method name or a subroutine reference, and the
   left side must be either an object (a blessed reference) or a class
   name (that is, a package name).  See perlobj.

   The dereferencing cases (as opposed to method-calling cases) are
   somewhat extended by the "postderef" feature.  For the details of that
   feature, consult "Postfix Dereference Syntax" in perlref.

   Auto-increment and Auto-decrement
   "++" and "--" work as in C.  That is, if placed before a variable, they
   increment or decrement the variable by one before returning the value,
   and if placed after, increment or decrement after returning the value.

       $i = 0;  $j = 0;
       print $i++;  # prints 0
       print ++$j;  # prints 1

   Note that just as in C, Perl doesn't define when the variable is
   incremented or decremented.  You just know it will be done sometime
   before or after the value is returned.  This also means that modifying
   a variable twice in the same statement will lead to undefined behavior.
   Avoid statements like:

       $i = $i ++;
       print ++ $i + $i ++;

   Perl will not guarantee what the result of the above statements is.

   The auto-increment operator has a little extra builtin magic to it.  If
   you increment a variable that is numeric, or that has ever been used in
   a numeric context, you get a normal increment.  If, however, the
   variable has been used in only string contexts since it was set, and
   has a value that is not the empty string and matches the pattern
   "/^[a-zA-Z]*[0-9]*\z/", the increment is done as a string, preserving
   each character within its range, with carry:

       print ++($foo = "99");      # prints "100"
       print ++($foo = "a0");      # prints "a1"
       print ++($foo = "Az");      # prints "Ba"
       print ++($foo = "zz");      # prints "aaa"

   "undef" is always treated as numeric, and in particular is changed to 0
   before incrementing (so that a post-increment of an undef value will
   return 0 rather than "undef").

   The auto-decrement operator is not magical.

   Exponentiation
   Binary "**" is the exponentiation operator.  It binds even more tightly
   than unary minus, so "-2**4" is "-(2**4)", not "(-2)**4".  (This is
   implemented using C's pow(3) function, which actually works on doubles
   internally.)

   Note that certain exponentiation expressions are ill-defined: these
   include "0**0", "1**Inf", and "Inf**0".  Do not expect any particular
   results from these special cases, the results are platform-dependent.

   Symbolic Unary Operators
   Unary "!" performs logical negation, that is, "not".  See also "not"
   for a lower precedence version of this.

   Unary "-" performs arithmetic negation if the operand is numeric,
   including any string that looks like a number.  If the operand is an
   identifier, a string consisting of a minus sign concatenated with the
   identifier is returned.  Otherwise, if the string starts with a plus or
   minus, a string starting with the opposite sign is returned.  One
   effect of these rules is that "-bareword" is equivalent to the string
   "-bareword".  If, however, the string begins with a non-alphabetic
   character (excluding "+" or "-"), Perl will attempt to convert the
   string to a numeric, and the arithmetic negation is performed.  If the
   string cannot be cleanly converted to a numeric, Perl will give the
   warning Argument "the string" isn't numeric in negation (-) at ....

   Unary "~" performs bitwise negation, that is, 1's complement.  For
   example, "0666&~027" is 0640.  (See also "Integer Arithmetic" and
   "Bitwise String Operators".)  Note that the width of the result is
   platform-dependent: "~0" is 32 bits wide on a 32-bit platform, but 64
   bits wide on a 64-bit platform, so if you are expecting a certain bit
   width, remember to use the "&" operator to mask off the excess bits.

   When complementing strings, if all characters have ordinal values under
   256, then their complements will, also.  But if they do not, all
   characters will be in either 32- or 64-bit complements, depending on
   your architecture.  So for example, "~"\x{3B1}"" is "\x{FFFF_FC4E}" on
   32-bit machines and "\x{FFFF_FFFF_FFFF_FC4E}" on 64-bit machines.

   If the experimental "bitwise" feature is enabled via
   "usefeature'bitwise'", then unary "~" always treats its argument as a
   number, and an alternate form of the operator, "~.", always treats its
   argument as a string.  So "~0" and "~"0"" will both give 2**32-1 on
   32-bit platforms, whereas "~.0" and "~."0"" will both yield "\xff".
   This feature produces a warning unless you use
   "nowarnings'experimental::bitwise'".

   Unary "+" has no effect whatsoever, even on strings.  It is useful
   syntactically for separating a function name from a parenthesized
   expression that would otherwise be interpreted as the complete list of
   function arguments.  (See examples above under "Terms and List
   Operators (Leftward)".)

   Unary "\" creates a reference to whatever follows it.  See perlreftut
   and perlref.  Do not confuse this behavior with the behavior of
   backslash within a string, although both forms do convey the notion of
   protecting the next thing from interpolation.

   Binding Operators
   Binary "=~" binds a scalar expression to a pattern match.  Certain
   operations search or modify the string $_ by default.  This operator
   makes that kind of operation work on some other string.  The right
   argument is a search pattern, substitution, or transliteration.  The
   left argument is what is supposed to be searched, substituted, or
   transliterated instead of the default $_.  When used in scalar context,
   the return value generally indicates the success of the operation.  The
   exceptions are substitution ("s///") and transliteration ("y///") with
   the "/r" (non-destructive) option, which cause the return value to be
   the result of the substitution.  Behavior in list context depends on
   the particular operator.  See "Regexp Quote-Like Operators" for details
   and perlretut for examples using these operators.

   If the right argument is an expression rather than a search pattern,
   substitution, or transliteration, it is interpreted as a search pattern
   at run time.  Note that this means that its contents will be
   interpolated twice, so

       '\\' =~ q'\\';

   is not ok, as the regex engine will end up trying to compile the
   pattern "\", which it will consider a syntax error.

   Binary "!~" is just like "=~" except the return value is negated in the
   logical sense.

   Binary "!~" with a non-destructive substitution ("s///r") or
   transliteration ("y///r") is a syntax error.

   Multiplicative Operators
   Binary "*" multiplies two numbers.

   Binary "/" divides two numbers.

   Binary "%" is the modulo operator, which computes the division
   remainder of its first argument with respect to its second argument.
   Given integer operands $m and $n: If $n is positive, then "$m%$n" is
   $m minus the largest multiple of $n less than or equal to $m.  If $n is
   negative, then "$m%$n" is $m minus the smallest multiple of $n that
   is not less than $m (that is, the result will be less than or equal to
   zero).  If the operands $m and $n are floating point values and the
   absolute value of $n (that is "abs($n)") is less than "(UV_MAX+1)",
   only the integer portion of $m and $n will be used in the operation
   (Note: here "UV_MAX" means the maximum of the unsigned integer type).
   If the absolute value of the right operand ("abs($n)") is greater than
   or equal to "(UV_MAX+1)", "%" computes the floating-point remainder
   $r in the equation "($r=$m-$i*$n)" where $i is a certain integer
   that makes $r have the same sign as the right operand $n (not as the
   left operand $m like C function "fmod()") and the absolute value less
   than that of $n.  Note that when "useinteger" is in scope, "%" gives
   you direct access to the modulo operator as implemented by your C
   compiler.  This operator is not as well defined for negative operands,
   but it will execute faster.

   Binary "x" is the repetition operator.  In scalar context or if the
   left operand is not enclosed in parentheses, it returns a string
   consisting of the left operand repeated the number of times specified
   by the right operand.  In list context, if the left operand is enclosed
   in parentheses or is a list formed by "qw/STRING/", it repeats the
   list.  If the right operand is zero or negative (raising a warning on
   negative), it returns an empty string or an empty list, depending on
   the context.

       print '-' x 80;             # print row of dashes

       print "\t" x ($tab/8), ' ' x ($tab%8);      # tab over

       @ones = (1) x 80;           # a list of 80 1's
       @ones = (5) x @ones;        # set all elements to 5

   Additive Operators
   Binary "+" returns the sum of two numbers.

   Binary "-" returns the difference of two numbers.

   Binary "." concatenates two strings.

   Shift Operators
   Binary "<<" returns the value of its left argument shifted left by the
   number of bits specified by the right argument.  Arguments should be
   integers.  (See also "Integer Arithmetic".)

   Binary ">>" returns the value of its left argument shifted right by the
   number of bits specified by the right argument.  Arguments should be
   integers.  (See also "Integer Arithmetic".)

   If "useinteger" (see "Integer Arithmetic") is in force then signed C
   integers are used (arithmetic shift), otherwise unsigned C integers are
   used (logical shift), even for negative shiftees.  In arithmetic right
   shift the sign bit is replicated on the left, in logical shift zero
   bits come in from the left.

   Either way, the implementation isn't going to generate results larger
   than the size of the integer type Perl was built with (32 bits or 64
   bits).

   Shifting by negative number of bits means the reverse shift: left shift
   becomes right shift, right shift becomes left shift.  This is unlike in
   C, where negative shift is undefined.

   Shifting by more bits than the size of the integers means most of the
   time zero (all bits fall off), except that under "useinteger" right
   overshifting a negative shiftee results in -1.  This is unlike in C,
   where shifting by too many bits is undefined.  A common C behavior is
   "shift by modulo wordbits", so that for example

       1 >> 64 == 1 >> (64 % 64) == 1 >> 0 == 1  # Common C behavior.

   but that is completely accidental.

   If you get tired of being subject to your platform's native integers,
   the "usebigint" pragma neatly sidesteps the issue altogether:

       print 20 << 20;  # 20971520
       print 20 << 40;  # 5120 on 32-bit machines,
                        # 21990232555520 on 64-bit machines
       use bigint;
       print 20 << 100; # 25353012004564588029934064107520

   Named Unary Operators
   The various named unary operators are treated as functions with one
   argument, with optional parentheses.

   If any list operator ("print()", etc.) or any unary operator
   ("chdir()", etc.)  is followed by a left parenthesis as the next token,
   the operator and arguments within parentheses are taken to be of
   highest precedence, just like a normal function call.  For example,
   because named unary operators are higher precedence than "||":

       chdir $foo    || die;       # (chdir $foo) || die
       chdir($foo)   || die;       # (chdir $foo) || die
       chdir ($foo)  || die;       # (chdir $foo) || die
       chdir +($foo) || die;       # (chdir $foo) || die

   but, because "*" is higher precedence than named operators:

       chdir $foo * 20;    # chdir ($foo * 20)
       chdir($foo) * 20;   # (chdir $foo) * 20
       chdir ($foo) * 20;  # (chdir $foo) * 20
       chdir +($foo) * 20; # chdir ($foo * 20)

       rand 10 * 20;       # rand (10 * 20)
       rand(10) * 20;      # (rand 10) * 20
       rand (10) * 20;     # (rand 10) * 20
       rand +(10) * 20;    # rand (10 * 20)

   Regarding precedence, the filetest operators, like "-f", "-M", etc. are
   treated like named unary operators, but they don't follow this
   functional parenthesis rule.  That means, for example, that
   "-f($file).".bak"" is equivalent to "-f"$file.bak"".

   See also "Terms and List Operators (Leftward)".

   Relational Operators
   Perl operators that return true or false generally return values that
   can be safely used as numbers.  For example, the relational operators
   in this section and the equality operators in the next one return 1 for
   true and a special version of the defined empty string, "", which
   counts as a zero but is exempt from warnings about improper numeric
   conversions, just as "0buttrue" is.

   Binary "<" returns true if the left argument is numerically less than
   the right argument.

   Binary ">" returns true if the left argument is numerically greater
   than the right argument.

   Binary "<=" returns true if the left argument is numerically less than
   or equal to the right argument.

   Binary ">=" returns true if the left argument is numerically greater
   than or equal to the right argument.

   Binary "lt" returns true if the left argument is stringwise less than
   the right argument.

   Binary "gt" returns true if the left argument is stringwise greater
   than the right argument.

   Binary "le" returns true if the left argument is stringwise less than
   or equal to the right argument.

   Binary "ge" returns true if the left argument is stringwise greater
   than or equal to the right argument.

   Equality Operators
   Binary "==" returns true if the left argument is numerically equal to
   the right argument.

   Binary "!=" returns true if the left argument is numerically not equal
   to the right argument.

   Binary "<=>" returns -1, 0, or 1 depending on whether the left argument
   is numerically less than, equal to, or greater than the right argument.
   If your platform supports "NaN"'s (not-a-numbers) as numeric values,
   using them with "<=>" returns undef.  "NaN" is not "<", "==", ">", "<="
   or ">=" anything (even "NaN"), so those 5 return false.  "NaN!=NaN"
   returns true, as does "NaN!="anythingelse.  If your platform doesn't
   support "NaN"'s then "NaN" is just a string with numeric value 0.

       $ perl -le '$x = "NaN"; print "No NaN support here" if $x == $x'
       $ perl -le '$x = "NaN"; print "NaN support here" if $x != $x'

   (Note that the bigint, bigrat, and bignum pragmas all support "NaN".)

   Binary "eq" returns true if the left argument is stringwise equal to
   the right argument.

   Binary "ne" returns true if the left argument is stringwise not equal
   to the right argument.

   Binary "cmp" returns -1, 0, or 1 depending on whether the left argument
   is stringwise less than, equal to, or greater than the right argument.

   Binary "~~" does a smartmatch between its arguments.  Smart matching is
   described in the next section.

   "lt", "le", "ge", "gt" and "cmp" use the collation (sort) order
   specified by the current "LC_COLLATE" locale if a "uselocale" form
   that includes collation is in effect.  See perllocale.  Do not mix
   these with Unicode, only use them with legacy 8-bit locale encodings.
   The standard "Unicode::Collate" and "Unicode::Collate::Locale" modules
   offer much more powerful solutions to collation issues.

   For case-insensitive comparisions, look at the "fc" in perlfunc case-
   folding function, available in Perl v5.16 or later:

       if ( fc($x) eq fc($y) ) { ... }

   Smartmatch Operator
   First available in Perl 5.10.1 (the 5.10.0 version behaved
   differently), binary "~~" does a "smartmatch" between its arguments.
   This is mostly used implicitly in the "when" construct described in
   perlsyn, although not all "when" clauses call the smartmatch operator.
   Unique among all of Perl's operators, the smartmatch operator can
   recurse.  The smartmatch operator is experimental and its behavior is
   subject to change.

   It is also unique in that all other Perl operators impose a context
   (usually string or numeric context) on their operands, autoconverting
   those operands to those imposed contexts.  In contrast, smartmatch
   infers contexts from the actual types of its operands and uses that
   type information to select a suitable comparison mechanism.

   The "~~" operator compares its operands "polymorphically", determining
   how to compare them according to their actual types (numeric, string,
   array, hash, etc.)  Like the equality operators with which it shares
   the same precedence, "~~" returns 1 for true and "" for false.  It is
   often best read aloud as "in", "inside of", or "is contained in",
   because the left operand is often looked for inside the right operand.
   That makes the order of the operands to the smartmatch operand often
   opposite that of the regular match operator.  In other words, the
   "smaller" thing is usually placed in the left operand and the larger
   one in the right.

   The behavior of a smartmatch depends on what type of things its
   arguments are, as determined by the following table.  The first row of
   the table whose types apply determines the smartmatch behavior.
   Because what actually happens is mostly determined by the type of the
   second operand, the table is sorted on the right operand instead of on
   the left.

    Left      Right      Description and pseudocode
    ===============================================================
    Any       undef      check whether Any is undefined
                   like: !defined Any

    Any       Object     invoke ~~ overloading on Object, or die

    Right operand is an ARRAY:

    Left      Right      Description and pseudocode
    ===============================================================
    ARRAY1    ARRAY2     recurse on paired elements of ARRAY1 and ARRAY2[2]
                   like: (ARRAY1[0] ~~ ARRAY2[0])
                           && (ARRAY1[1] ~~ ARRAY2[1]) && ...
    HASH      ARRAY      any ARRAY elements exist as HASH keys
                   like: grep { exists HASH->{$_} } ARRAY
    Regexp    ARRAY      any ARRAY elements pattern match Regexp
                   like: grep { /Regexp/ } ARRAY
    undef     ARRAY      undef in ARRAY
                   like: grep { !defined } ARRAY
    Any       ARRAY      smartmatch each ARRAY element[3]
                   like: grep { Any ~~ $_ } ARRAY

    Right operand is a HASH:

    Left      Right      Description and pseudocode
    ===============================================================
    HASH1     HASH2      all same keys in both HASHes
                   like: keys HASH1 ==
                            grep { exists HASH2->{$_} } keys HASH1
    ARRAY     HASH       any ARRAY elements exist as HASH keys
                   like: grep { exists HASH->{$_} } ARRAY
    Regexp    HASH       any HASH keys pattern match Regexp
                   like: grep { /Regexp/ } keys HASH
    undef     HASH       always false (undef can't be a key)
                   like: 0 == 1
    Any       HASH       HASH key existence
                   like: exists HASH->{Any}

    Right operand is CODE:

    Left      Right      Description and pseudocode
    ===============================================================
    ARRAY     CODE       sub returns true on all ARRAY elements[1]
                   like: !grep { !CODE->($_) } ARRAY
    HASH      CODE       sub returns true on all HASH keys[1]
                   like: !grep { !CODE->($_) } keys HASH
    Any       CODE       sub passed Any returns true
                   like: CODE->(Any)

   Right operand is a Regexp:

    Left      Right      Description and pseudocode
    ===============================================================
    ARRAY     Regexp     any ARRAY elements match Regexp
                   like: grep { /Regexp/ } ARRAY
    HASH      Regexp     any HASH keys match Regexp
                   like: grep { /Regexp/ } keys HASH
    Any       Regexp     pattern match
                   like: Any =~ /Regexp/

    Other:

    Left      Right      Description and pseudocode
    ===============================================================
    Object    Any        invoke ~~ overloading on Object,
                         or fall back to...

    Any       Num        numeric equality
                    like: Any == Num
    Num       nummy[4]    numeric equality
                    like: Num == nummy
    undef     Any        check whether undefined
                    like: !defined(Any)
    Any       Any        string equality
                    like: Any eq Any

   Notes:

   1. Empty hashes or arrays match.
   2. That is, each element smartmatches the element of the same index in
   the other array.[3]
   3. If a circular reference is found, fall back to referential equality.
   4. Either an actual number, or a string that looks like one.

   The smartmatch implicitly dereferences any non-blessed hash or array
   reference, so the "HASH" and "ARRAY" entries apply in those cases.  For
   blessed references, the "Object" entries apply.  Smartmatches involving
   hashes only consider hash keys, never hash values.

   The "like" code entry is not always an exact rendition.  For example,
   the smartmatch operator short-circuits whenever possible, but "grep"
   does not.  Also, "grep" in scalar context returns the number of
   matches, but "~~" returns only true or false.

   Unlike most operators, the smartmatch operator knows to treat "undef"
   specially:

       use v5.10.1;
       @array = (1, 2, 3, undef, 4, 5);
       say "some elements undefined" if undef ~~ @array;

   Each operand is considered in a modified scalar context, the
   modification being that array and hash variables are passed by
   reference to the operator, which implicitly dereferences them.  Both
   elements of each pair are the same:

       use v5.10.1;

       my %hash = (red    => 1, blue   => 2, green  => 3,
                   orange => 4, yellow => 5, purple => 6,
                   black  => 7, grey   => 8, white  => 9);

       my @array = qw(red blue green);

       say "some array elements in hash keys" if  @array ~~  %hash;
       say "some array elements in hash keys" if \@array ~~ \%hash;

       say "red in array" if "red" ~~  @array;
       say "red in array" if "red" ~~ \@array;

       say "some keys end in e" if /e$/ ~~  %hash;
       say "some keys end in e" if /e$/ ~~ \%hash;

   Two arrays smartmatch if each element in the first array smartmatches
   (that is, is "in") the corresponding element in the second array,
   recursively.

       use v5.10.1;
       my @little = qw(red blue green);
       my @bigger = ("red", "blue", [ "orange", "green" ] );
       if (@little ~~ @bigger) {  # true!
           say "little is contained in bigger";
       }

   Because the smartmatch operator recurses on nested arrays, this will
   still report that "red" is in the array.

       use v5.10.1;
       my @array = qw(red blue green);
       my $nested_array = [[[[[[[ @array ]]]]]]];
       say "red in array" if "red" ~~ $nested_array;

   If two arrays smartmatch each other, then they are deep copies of each
   others' values, as this example reports:

       use v5.12.0;
       my @a = (0, 1, 2, [3, [4, 5], 6], 7);
       my @b = (0, 1, 2, [3, [4, 5], 6], 7);

       if (@a ~~ @b && @b ~~ @a) {
           say "a and b are deep copies of each other";
       }
       elsif (@a ~~ @b) {
           say "a smartmatches in b";
       }
       elsif (@b ~~ @a) {
           say "b smartmatches in a";
       }
       else {
           say "a and b don't smartmatch each other at all";
       }

   If you were to set "$b[3]=4", then instead of reporting that "a and b
   are deep copies of each other", it now reports that "b smartmatches in
   a".  That's because the corresponding position in @a contains an array
   that (eventually) has a 4 in it.

   Smartmatching one hash against another reports whether both contain the
   same keys, no more and no less.  This could be used to see whether two
   records have the same field names, without caring what values those
   fields might have.  For example:

       use v5.10.1;
       sub make_dogtag {
           state $REQUIRED_FIELDS = { name=>1, rank=>1, serial_num=>1 };

           my ($class, $init_fields) = @_;

           die "Must supply (only) name, rank, and serial number"
               unless $init_fields ~~ $REQUIRED_FIELDS;

           ...
       }

   However, this only does what you mean if $init_fields is indeed a hash
   reference. The condition "$init_fields ~~ $REQUIRED_FIELDS" also allows
   the strings "name", "rank", "serial_num" as well as any array reference
   that contains "name" or "rank" or "serial_num" anywhere to pass
   through.

   The smartmatch operator is most often used as the implicit operator of
   a "when" clause.  See the section on "Switch Statements" in perlsyn.

   Smartmatching of Objects

   To avoid relying on an object's underlying representation, if the
   smartmatch's right operand is an object that doesn't overload "~~", it
   raises the exception ""Smartmatching a non-overloaded object breaks
   encapsulation"".  That's because one has no business digging around to
   see whether something is "in" an object.  These are all illegal on
   objects without a "~~" overload:

       %hash ~~ $object
          42 ~~ $object
      "fred" ~~ $object

   However, you can change the way an object is smartmatched by
   overloading the "~~" operator.  This is allowed to extend the usual
   smartmatch semantics.  For objects that do have an "~~" overload, see
   overload.

   Using an object as the left operand is allowed, although not very
   useful.  Smartmatching rules take precedence over overloading, so even
   if the object in the left operand has smartmatch overloading, this will
   be ignored.  A left operand that is a non-overloaded object falls back
   on a string or numeric comparison of whatever the "ref" operator
   returns.  That means that

       $object ~~ X

   does not invoke the overload method with "X" as an argument.  Instead
   the above table is consulted as normal, and based on the type of "X",
   overloading may or may not be invoked.  For simple strings or numbers,
   "in" becomes equivalent to this:

       $object ~~ $number          ref($object) == $number
       $object ~~ $string          ref($object) eq $string

   For example, this reports that the handle smells IOish (but please
   don't really do this!):

       use IO::Handle;
       my $fh = IO::Handle->new();
       if ($fh ~~ /	IO	/) {
           say "handle smells IOish";
       }

   That's because it treats $fh as a string like
   "IO::Handle=GLOB(0x8039e0)", then pattern matches against that.

   Bitwise And
   Binary "&" returns its operands ANDed together bit by bit.  Although no
   warning is currently raised, the result is not well defined when this
   operation is performed on operands that aren't either numbers (see
   "Integer Arithmetic") nor bitstrings (see "Bitwise String Operators").

   Note that "&" has lower priority than relational operators, so for
   example the parentheses are essential in a test like

       print "Even\n" if ($x & 1) == 0;

   If the experimental "bitwise" feature is enabled via
   "usefeature'bitwise'", then this operator always treats its operand
   as numbers.  This feature produces a warning unless you also use
   "nowarnings'experimental::bitwise'".

   Bitwise Or and Exclusive Or
   Binary "|" returns its operands ORed together bit by bit.

   Binary "^" returns its operands XORed together bit by bit.

   Although no warning is currently raised, the results are not well
   defined when these operations are performed on operands that aren't
   either numbers (see "Integer Arithmetic") nor bitstrings (see "Bitwise
   String Operators").

   Note that "|" and "^" have lower priority than relational operators, so
   for example the parentheses are essential in a test like

       print "false\n" if (8 | 2) != 10;

   If the experimental "bitwise" feature is enabled via
   "usefeature'bitwise'", then this operator always treats its operand
   as numbers.  This feature produces a warning unless you also use
   "nowarnings'experimental::bitwise'".

   C-style Logical And
   Binary "&&" performs a short-circuit logical AND operation.  That is,
   if the left operand is false, the right operand is not even evaluated.
   Scalar or list context propagates down to the right operand if it is
   evaluated.

   C-style Logical Or
   Binary "||" performs a short-circuit logical OR operation.  That is, if
   the left operand is true, the right operand is not even evaluated.
   Scalar or list context propagates down to the right operand if it is
   evaluated.

   Logical Defined-Or
   Although it has no direct equivalent in C, Perl's "//" operator is
   related to its C-style "or".  In fact, it's exactly the same as "||",
   except that it tests the left hand side's definedness instead of its
   truth.  Thus, "EXPR1//EXPR2" returns the value of "EXPR1" if it's
   defined, otherwise, the value of "EXPR2" is returned.  ("EXPR1" is
   evaluated in scalar context, "EXPR2" in the context of "//" itself).
   Usually, this is the same result as "defined(EXPR1)?EXPR1:EXPR2"
   (except that the ternary-operator form can be used as a lvalue, while
   "EXPR1//EXPR2" cannot).  This is very useful for providing default
   values for variables.  If you actually want to test if at least one of
   $x and $y is defined, use "defined($x//$y)".

   The "||", "//" and "&&" operators return the last value evaluated
   (unlike C's "||" and "&&", which return 0 or 1).  Thus, a reasonably
   portable way to find out the home directory might be:

       $home =  $ENV{HOME}
             // $ENV{LOGDIR}
             // (getpwuid($<))[7]
             // die "You're homeless!\n";

   In particular, this means that you shouldn't use this for selecting
   between two aggregates for assignment:

       @a = @b || @c;            # This doesn't do the right thing
       @a = scalar(@b) || @c;    # because it really means this.
       @a = @b ? @b : @c;        # This works fine, though.

   As alternatives to "&&" and "||" when used for control flow, Perl
   provides the "and" and "or" operators (see below).  The short-circuit
   behavior is identical.  The precedence of "and" and "or" is much lower,
   however, so that you can safely use them after a list operator without
   the need for parentheses:

       unlink "alpha", "beta", "gamma"
               or gripe(), next LINE;

   With the C-style operators that would have been written like this:

       unlink("alpha", "beta", "gamma")
               || (gripe(), next LINE);

   It would be even more readable to write that this way:

       unless(unlink("alpha", "beta", "gamma")) {
           gripe();
           next LINE;
       }

   Using "or" for assignment is unlikely to do what you want; see below.

   Range Operators
   Binary ".." is the range operator, which is really two different
   operators depending on the context.  In list context, it returns a list
   of values counting (up by ones) from the left value to the right value.
   If the left value is greater than the right value then it returns the
   empty list.  The range operator is useful for writing "foreach(1..10)"
   loops and for doing slice operations on arrays.  In the current
   implementation, no temporary array is created when the range operator
   is used as the expression in "foreach" loops, but older versions of
   Perl might burn a lot of memory when you write something like this:

       for (1 .. 1_000_000) {
           # code
       }

   The range operator also works on strings, using the magical auto-
   increment, see below.

   In scalar context, ".." returns a boolean value.  The operator is
   bistable, like a flip-flop, and emulates the line-range (comma)
   operator of sed, awk, and various editors.  Each ".." operator
   maintains its own boolean state, even across calls to a subroutine that
   contains it.  It is false as long as its left operand is false.  Once
   the left operand is true, the range operator stays true until the right
   operand is true, AFTER which the range operator becomes false again.
   It doesn't become false till the next time the range operator is
   evaluated.  It can test the right operand and become false on the same
   evaluation it became true (as in awk), but it still returns true once.
   If you don't want it to test the right operand until the next
   evaluation, as in sed, just use three dots ("...") instead of two.  In
   all other regards, "..." behaves just like ".." does.

   The right operand is not evaluated while the operator is in the "false"
   state, and the left operand is not evaluated while the operator is in
   the "true" state.  The precedence is a little lower than || and &&.
   The value returned is either the empty string for false, or a sequence
   number (beginning with 1) for true.  The sequence number is reset for
   each range encountered.  The final sequence number in a range has the
   string "E0" appended to it, which doesn't affect its numeric value, but
   gives you something to search for if you want to exclude the endpoint.
   You can exclude the beginning point by waiting for the sequence number
   to be greater than 1.

   If either operand of scalar ".." is a constant expression, that operand
   is considered true if it is equal ("==") to the current input line
   number (the $. variable).

   To be pedantic, the comparison is actually "int(EXPR)==int(EXPR)",
   but that is only an issue if you use a floating point expression; when
   implicitly using $. as described in the previous paragraph, the
   comparison is "int(EXPR)==int($.)" which is only an issue when $.  is
   set to a floating point value and you are not reading from a file.
   Furthermore, "span".."spat" or "2.18..3.14" will not do what you
   want in scalar context because each of the operands are evaluated using
   their integer representation.

   Examples:

   As a scalar operator:

       if (101 .. 200) { print; } # print 2nd hundred lines, short for
                                  #  if ($. == 101 .. $. == 200) { print; }

       next LINE if (1 .. /^$/);  # skip header lines, short for
                                  #   next LINE if ($. == 1 .. /^$/);
                                  # (typically in a loop labeled LINE)

       s/^/> / if (/^$/ .. eof());  # quote body

       # parse mail messages
       while (<>) {
           $in_header =   1  .. /^$/;
           $in_body   = /^$/ .. eof;
           if ($in_header) {
               # do something
           } else { # in body
               # do something else
           }
       } continue {
           close ARGV if eof;             # reset $. each file
       }

   Here's a simple example to illustrate the difference between the two
   range operators:

       @lines = ("   - Foo",
                 "01 - Bar",
                 "1  - Baz",
                 "   - Quux");

       foreach (@lines) {
           if (/0/ .. /1/) {
               print "$_\n";
           }
       }

   This program will print only the line containing "Bar".  If the range
   operator is changed to "...", it will also print the "Baz" line.

   And now some examples as a list operator:

       for (101 .. 200) { print }      # print $_ 100 times
       @foo = @foo[0 .. $#foo];        # an expensive no-op
       @foo = @foo[$#foo-4 .. $#foo];  # slice last 5 items

   The range operator (in list context) makes use of the magical auto-
   increment algorithm if the operands are strings.  You can say

       @alphabet = ("A" .. "Z");

   to get all normal letters of the English alphabet, or

       $hexdigit = (0 .. 9, "a" .. "f")[$num & 15];

   to get a hexadecimal digit, or

       @z2 = ("01" .. "31");
       print $z2[$mday];

   to get dates with leading zeros.

   If the final value specified is not in the sequence that the magical
   increment would produce, the sequence goes until the next value would
   be longer than the final value specified.

   If the initial value specified isn't part of a magical increment
   sequence (that is, a non-empty string matching "/^[a-zA-Z]*[0-9]*\z/"),
   only the initial value will be returned.  So the following will only
   return an alpha:

       use charnames "greek";
       my @greek_small =  ("\N{alpha}" .. "\N{omega}");

   To get the 25 traditional lowercase Greek letters, including both
   sigmas, you could use this instead:

       use charnames "greek";
       my @greek_small =  map { chr } ( ord("\N{alpha}")
                                           ..
                                        ord("\N{omega}")
                                      );

   However, because there are many other lowercase Greek characters than
   just those, to match lowercase Greek characters in a regular
   expression, you could use the pattern "/(?:(?=\p{Greek})\p{Lower})+/"
   (or the experimental feature "/(?[\p{Greek}&\p{Lower}])+/").

   Because each operand is evaluated in integer form, "2.18..3.14" will
   return two elements in list context.

       @list = (2.18 .. 3.14); # same as @list = (2 .. 3);

   Conditional Operator
   Ternary "?:" is the conditional operator, just as in C.  It works much
   like an if-then-else.  If the argument before the "?" is true, the
   argument before the ":" is returned, otherwise the argument after the
   ":" is returned.  For example:

       printf "I have %d dog%s.\n", $n,
               ($n == 1) ? "" : "s";

   Scalar or list context propagates downward into the 2nd or 3rd
   argument, whichever is selected.

       $x = $ok ? $y : $z;  # get a scalar
       @x = $ok ? @y : @z;  # get an array
       $x = $ok ? @y : @z;  # oops, that's just a count!

   The operator may be assigned to if both the 2nd and 3rd arguments are
   legal lvalues (meaning that you can assign to them):

       ($x_or_y ? $x : $y) = $z;

   Because this operator produces an assignable result, using assignments
   without parentheses will get you in trouble.  For example, this:

       $x % 2 ? $x += 10 : $x += 2

   Really means this:

       (($x % 2) ? ($x += 10) : $x) += 2

   Rather than this:

       ($x % 2) ? ($x += 10) : ($x += 2)

   That should probably be written more simply as:

       $x += ($x % 2) ? 10 : 2;

   Assignment Operators
   "=" is the ordinary assignment operator.

   Assignment operators work as in C.  That is,

       $x += 2;

   is equivalent to

       $x = $x + 2;

   although without duplicating any side effects that dereferencing the
   lvalue might trigger, such as from "tie()".  Other assignment operators
   work similarly.  The following are recognized:

       **=    +=    *=    &=    &.=    <<=    &&=
              -=    /=    |=    |.=    >>=    ||=
              .=    %=    ^=    ^.=           //=
                    x=

   Although these are grouped by family, they all have the precedence of
   assignment.  These combined assignment operators can only operate on
   scalars, whereas the ordinary assignment operator can assign to arrays,
   hashes, lists and even references.  (See "Context" and "List value
   constructors" in perldata, and "Assigning to References" in perlref.)

   Unlike in C, the scalar assignment operator produces a valid lvalue.
   Modifying an assignment is equivalent to doing the assignment and then
   modifying the variable that was assigned to.  This is useful for
   modifying a copy of something, like this:

       ($tmp = $global) =~ tr/13579/24680/;

   Although as of 5.14, that can be also be accomplished this way:

       use v5.14;
       $tmp = ($global =~  tr/13579/24680/r);

   Likewise,

       ($x += 2) *= 3;

   is equivalent to

       $x += 2;
       $x *= 3;

   Similarly, a list assignment in list context produces the list of
   lvalues assigned to, and a list assignment in scalar context returns
   the number of elements produced by the expression on the right hand
   side of the assignment.

   The three dotted bitwise assignment operators ("&.=" "|.=" "^.=") are
   new in Perl 5.22 and experimental.  See "Bitwise String Operators".

   Comma Operator
   Binary "," is the comma operator.  In scalar context it evaluates its
   left argument, throws that value away, then evaluates its right
   argument and returns that value.  This is just like C's comma operator.

   In list context, it's just the list argument separator, and inserts
   both its arguments into the list.  These arguments are also evaluated
   from left to right.

   The "=>" operator (sometimes pronounced "fat comma") is a synonym for
   the comma except that it causes a word on its left to be interpreted as
   a string if it begins with a letter or underscore and is composed only
   of letters, digits and underscores.  This includes operands that might
   otherwise be interpreted as operators, constants, single number
   v-strings or function calls.  If in doubt about this behavior, the left
   operand can be quoted explicitly.

   Otherwise, the "=>" operator behaves exactly as the comma operator or
   list argument separator, according to context.

   For example:

       use constant FOO => "something";

       my %h = ( FOO => 23 );

   is equivalent to:

       my %h = ("FOO", 23);

   It is NOT:

       my %h = ("something", 23);

   The "=>" operator is helpful in documenting the correspondence between
   keys and values in hashes, and other paired elements in lists.

       %hash = ( $key => $value );
       login( $username => $password );

   The special quoting behavior ignores precedence, and hence may apply to
   part of the left operand:

       print time.shift => "bbb";

   That example prints something like "1314363215shiftbbb", because the
   "=>" implicitly quotes the "shift" immediately on its left, ignoring
   the fact that "time.shift" is the entire left operand.

   List Operators (Rightward)
   On the right side of a list operator, the comma has very low
   precedence, such that it controls all comma-separated expressions found
   there.  The only operators with lower precedence are the logical
   operators "and", "or", and "not", which may be used to evaluate calls
   to list operators without the need for parentheses:

       open HANDLE, "< :utf8", "filename" or die "Can't open: $!\n";

   However, some people find that code harder to read than writing it with
   parentheses:

       open(HANDLE, "< :utf8", "filename") or die "Can't open: $!\n";

   in which case you might as well just use the more customary "||"
   operator:

       open(HANDLE, "< :utf8", "filename") || die "Can't open: $!\n";

   See also discussion of list operators in "Terms and List Operators
   (Leftward)".

   Logical Not
   Unary "not" returns the logical negation of the expression to its
   right.  It's the equivalent of "!" except for the very low precedence.

   Logical And
   Binary "and" returns the logical conjunction of the two surrounding
   expressions.  It's equivalent to "&&" except for the very low
   precedence.  This means that it short-circuits: the right expression is
   evaluated only if the left expression is true.

   Logical or and Exclusive Or
   Binary "or" returns the logical disjunction of the two surrounding
   expressions.  It's equivalent to "||" except for the very low
   precedence.  This makes it useful for control flow:

       print FH $data              or die "Can't write to FH: $!";

   This means that it short-circuits: the right expression is evaluated
   only if the left expression is false.  Due to its precedence, you must
   be careful to avoid using it as replacement for the "||" operator.  It
   usually works out better for flow control than in assignments:

       $x = $y or $z;              # bug: this is wrong
       ($x = $y) or $z;            # really means this
       $x = $y || $z;              # better written this way

   However, when it's a list-context assignment and you're trying to use
   "||" for control flow, you probably need "or" so that the assignment
   takes higher precedence.

       @info = stat($file) || die;     # oops, scalar sense of stat!
       @info = stat($file) or die;     # better, now @info gets its due

   Then again, you could always use parentheses.

   Binary "xor" returns the exclusive-OR of the two surrounding
   expressions.  It cannot short-circuit (of course).

   There is no low precedence operator for defined-OR.

   C Operators Missing From Perl
   Here is what C has that Perl doesn't:

   unary & Address-of operator.  (But see the "\" operator for taking a
           reference.)

   unary * Dereference-address operator.  (Perl's prefix dereferencing
           operators are typed: "$", "@", "%", and "&".)

   (TYPE)  Type-casting operator.

   Quote and Quote-like Operators
   While we usually think of quotes as literal values, in Perl they
   function as operators, providing various kinds of interpolating and
   pattern matching capabilities.  Perl provides customary quote
   characters for these behaviors, but also provides a way for you to
   choose your quote character for any of them.  In the following table, a
   "{}" represents any pair of delimiters you choose.

       Customary  Generic        Meaning        Interpolates
           ''       q{}          Literal             no
           ""      qq{}          Literal             yes
           ``      qx{}          Command             yes*
                   qw{}         Word list            no
           //       m{}       Pattern match          yes*
                   qr{}          Pattern             yes*
                    s{}{}      Substitution          yes*
                   tr{}{}    Transliteration         no (but see below)
                    y{}{}    Transliteration         no (but see below)
           <<EOF                 here-doc            yes*

           * unless the delimiter is ''.

   Non-bracketing delimiters use the same character fore and aft, but the
   four sorts of ASCII brackets (round, angle, square, curly) all nest,
   which means that

       q{foo{bar}baz}

   is the same as

       'foo{bar}baz'

   Note, however, that this does not always work for quoting Perl code:

       $s = q{ if($x eq "}") ... }; # WRONG

   is a syntax error.  The "Text::Balanced" module (standard as of v5.8,
   and from CPAN before then) is able to do this properly.

   There can be whitespace between the operator and the quoting
   characters, except when "#" is being used as the quoting character.
   "q#foo#" is parsed as the string "foo", while "q#foo#" is the operator
   "q" followed by a comment.  Its argument will be taken from the next
   line.  This allows you to write:

       s {foo}  # Replace foo
         {bar}  # with bar.

   The following escape sequences are available in constructs that
   interpolate, and in transliterations:

       Sequence     Note  Description
       \t                  tab               (HT, TAB)
       \n                  newline           (NL)
       \r                  return            (CR)
       \f                  form feed         (FF)
       	                  backspace         (BS)
       
                  alarm (bell)      (BEL)
       \e                  escape            (ESC)
       \x{263A}     [1,8]  hex char          (example: SMILEY)
       \x1b         [2,8]  restricted range hex char (example: ESC)
       \N{name}     [3]    named Unicode character or character sequence
       \N{U+263D}   [4,8]  Unicode character (example: FIRST QUARTER MOON)
       \c[          [5]    control char      (example: chr(27))
       \o{23072}    [6,8]  octal char        (example: SMILEY)
       \033         [7,8]  restricted range octal char  (example: ESC)

   [1] The result is the character specified by the hexadecimal number
       between the braces.  See "[8]" below for details on which
       character.

       Only hexadecimal digits are valid between the braces.  If an
       invalid character is encountered, a warning will be issued and the
       invalid character and all subsequent characters (valid or invalid)
       within the braces will be discarded.

       If there are no valid digits between the braces, the generated
       character is the NULL character ("\x{00}").  However, an explicit
       empty brace ("\x{}") will not cause a warning (currently).

   [2] The result is the character specified by the hexadecimal number in
       the range 0x00 to 0xFF.  See "[8]" below for details on which
       character.

       Only hexadecimal digits are valid following "\x".  When "\x" is
       followed by fewer than two valid digits, any valid digits will be
       zero-padded.  This means that "\x7" will be interpreted as "\x07",
       and a lone "\x" will be interpreted as "\x00".  Except at the end
       of a string, having fewer than two valid digits will result in a
       warning.  Note that although the warning says the illegal character
       is ignored, it is only ignored as part of the escape and will still
       be used as the subsequent character in the string.  For example:

         Original    Result    Warns?
         "\x7"       "\x07"    no
         "\x"        "\x00"    no
         "\x7q"      "\x07q"   yes
         "\xq"       "\x00q"   yes

   [3] The result is the Unicode character or character sequence given by
       name.  See charnames.

   [4] "\N{U+hexadecimalnumber}" means the Unicode character whose
       Unicode code point is hexadecimal number.

   [5] The character following "\c" is mapped to some other character as
       shown in the table:

        Sequence   Value
          \c@      chr(0)
          \cA      chr(1)
          \ca      chr(1)
          \cB      chr(2)
          \cb      chr(2)
          ...
          \cZ      chr(26)
          \cz      chr(26)
          \c[      chr(27)
                            # See below for chr(28)
          \c]      chr(29)
          \c^      chr(30)
          \c_      chr(31)
          \c?      chr(127) # (on ASCII platforms; see below for link to
                            #  EBCDIC discussion)

       In other words, it's the character whose code point has had 64
       xor'd with its uppercase.  "\c?" is DELETE on ASCII platforms
       because "ord("?")^64" is 127, and "\c@" is NULL because the ord
       of "@" is 64, so xor'ing 64 itself produces 0.

       Also, "\c\X" yields "chr(28)."X"" for any X, but cannot come at
       the end of a string, because the backslash would be parsed as
       escaping the end quote.

       On ASCII platforms, the resulting characters from the list above
       are the complete set of ASCII controls.  This isn't the case on
       EBCDIC platforms; see "OPERATOR DIFFERENCES" in perlebcdic for a
       full discussion of the differences between these for ASCII versus
       EBCDIC platforms.

       Use of any other character following the "c" besides those listed
       above is discouraged, and as of Perl v5.20, the only characters
       actually allowed are the printable ASCII ones, minus the left brace
       "{".  What happens for any of the allowed other characters is that
       the value is derived by xor'ing with the seventh bit, which is 64,
       and a warning raised if enabled.  Using the non-allowed characters
       generates a fatal error.

       To get platform independent controls, you can use "\N{...}".

   [6] The result is the character specified by the octal number between
       the braces.  See "[8]" below for details on which character.

       If a character that isn't an octal digit is encountered, a warning
       is raised, and the value is based on the octal digits before it,
       discarding it and all following characters up to the closing brace.
       It is a fatal error if there are no octal digits at all.

   [7] The result is the character specified by the three-digit octal
       number in the range 000 to 777 (but best to not use above 077, see
       next paragraph).  See "[8]" below for details on which character.

       Some contexts allow 2 or even 1 digit, but any usage without
       exactly three digits, the first being a zero, may give unintended
       results.  (For example, in a regular expression it may be confused
       with a backreference; see "Octal escapes" in perlrebackslash.)
       Starting in Perl 5.14, you may use "\o{}" instead, which avoids all
       these problems.  Otherwise, it is best to use this construct only
       for ordinals "\077" and below, remembering to pad to the left with
       zeros to make three digits.  For larger ordinals, either use
       "\o{}", or convert to something else, such as to hex and use
       "\N{U+}" (which is portable between platforms with different
       character sets) or "\x{}" instead.

   [8] Several constructs above specify a character by a number.  That
       number gives the character's position in the character set encoding
       (indexed from 0).  This is called synonymously its ordinal, code
       position, or code point.  Perl works on platforms that have a
       native encoding currently of either ASCII/Latin1 or EBCDIC, each of
       which allow specification of 256 characters.  In general, if the
       number is 255 (0xFF, 0377) or below, Perl interprets this in the
       platform's native encoding.  If the number is 256 (0x100, 0400) or
       above, Perl interprets it as a Unicode code point and the result is
       the corresponding Unicode character.  For example "\x{50}" and
       "\o{120}" both are the number 80 in decimal, which is less than
       256, so the number is interpreted in the native character set
       encoding.  In ASCII the character in the 80th position (indexed
       from 0) is the letter "P", and in EBCDIC it is the ampersand symbol
       "&".  "\x{100}" and "\o{400}" are both 256 in decimal, so the
       number is interpreted as a Unicode code point no matter what the
       native encoding is.  The name of the character in the 256th
       position (indexed by 0) in Unicode is "LATIN CAPITAL LETTER A WITH
       MACRON".

       There are a couple of exceptions to the above rule.
       "\N{U+hexnumber}" is always interpreted as a Unicode code point,
       so that "\N{U+0050}" is "P" even on EBCDIC platforms.  And if
       "useencoding" is in effect, the number is considered to be in that
       encoding, and is translated from that into the platform's native
       encoding if there is a corresponding native character; otherwise to
       Unicode.

   NOTE: Unlike C and other languages, Perl has no "\v" escape sequence
   for the vertical tab (VT, which is 11 in both ASCII and EBCDIC), but
   you may use "\N{VT}", "\ck", "\N{U+0b}", or "\x0b".  ("\v" does have
   meaning in regular expression patterns in Perl, see perlre.)

   The following escape sequences are available in constructs that
   interpolate, but not in transliterations.

       \l          lowercase next character only
       \u          titlecase (not uppercase!) next character only
       \L          lowercase all characters till \E or end of string
       \U          uppercase all characters till \E or end of string
       \F          foldcase all characters till \E or end of string
       \Q          quote (disable) pattern metacharacters till \E or
                   end of string
       \E          end either case modification or quoted section
                   (whichever was last seen)

   See "quotemeta" in perlfunc for the exact definition of characters that
   are quoted by "\Q".

   "\L", "\U", "\F", and "\Q" can stack, in which case you need one "\E"
   for each.  For example:

    say"This \Qquoting \ubusiness \Uhere isn't quite\E done yet,\E is it?";
    This quoting\ Business\ HERE\ ISN\'T\ QUITE\ done\ yet\, is it?

   If a "uselocale" form that includes "LC_CTYPE" is in effect (see
   perllocale), the case map used by "\l", "\L", "\u", and "\U" is taken
   from the current locale.  If Unicode (for example, "\N{}" or code
   points of 0x100 or beyond) is being used, the case map used by "\l",
   "\L", "\u", and "\U" is as defined by Unicode.  That means that case-
   mapping a single character can sometimes produce a sequence of several
   characters.  Under "uselocale", "\F" produces the same results as "\L"
   for all locales but a UTF-8 one, where it instead uses the Unicode
   definition.

   All systems use the virtual "\n" to represent a line terminator, called
   a "newline".  There is no such thing as an unvarying, physical newline
   character.  It is only an illusion that the operating system, device
   drivers, C libraries, and Perl all conspire to preserve.  Not all
   systems read "\r" as ASCII CR and "\n" as ASCII LF.  For example, on
   the ancient Macs (pre-MacOS X) of yesteryear, these used to be
   reversed, and on systems without a line terminator, printing "\n" might
   emit no actual data.  In general, use "\n" when you mean a "newline"
   for your system, but use the literal ASCII when you need an exact
   character.  For example, most networking protocols expect and prefer a
   CR+LF ("\015\012" or "\cM\cJ") for line terminators, and although they
   often accept just "\012", they seldom tolerate just "\015".  If you get
   in the habit of using "\n" for networking, you may be burned some day.

   For constructs that do interpolate, variables beginning with ""$"" or
   ""@"" are interpolated.  Subscripted variables such as $a[3] or
   "$href->{key}[0]" are also interpolated, as are array and hash slices.
   But method calls such as "$obj->meth" are not.

   Interpolating an array or slice interpolates the elements in order,
   separated by the value of $", so is equivalent to interpolating
   "join$",@array".  "Punctuation" arrays such as "@*" are usually
   interpolated only if the name is enclosed in braces "@{*}", but the
   arrays @_, "@+", and "@-" are interpolated even without braces.

   For double-quoted strings, the quoting from "\Q" is applied after
   interpolation and escapes are processed.

       "abc\Qfoo\tbar$s\Exyz"

   is equivalent to

       "abc" . quotemeta("foo\tbar$s") . "xyz"

   For the pattern of regex operators ("qr//", "m//" and "s///"), the
   quoting from "\Q" is applied after interpolation is processed, but
   before escapes are processed.  This allows the pattern to match
   literally (except for "$" and "@").  For example, the following
   matches:

       '\s\t' =~ /\Q\s\t/

   Because "$" or "@" trigger interpolation, you'll need to use something
   like "/\Quser\E\@\Qhost/" to match them literally.

   Patterns are subject to an additional level of interpretation as a
   regular expression.  This is done as a second pass, after variables are
   interpolated, so that regular expressions may be incorporated into the
   pattern from the variables.  If this is not what you want, use "\Q" to
   interpolate a variable literally.

   Apart from the behavior described above, Perl does not expand multiple
   levels of interpolation.  In particular, contrary to the expectations
   of shell programmers, back-quotes do NOT interpolate within double
   quotes, nor do single quotes impede evaluation of variables when used
   within double quotes.

   Regexp Quote-Like Operators
   Here are the quote-like operators that apply to pattern matching and
   related activities.

   "qr/STRING/msixpodualn"
           This operator quotes (and possibly compiles) its STRING as a
           regular expression.  STRING is interpolated the same way as
           PATTERN in "m/PATTERN/".  If "'" is used as the delimiter, no
           interpolation is done.  Returns a Perl value which may be used
           instead of the corresponding "/STRING/msixpodualn" expression.
           The returned value is a normalized version of the original
           pattern.  It magically differs from a string containing the
           same characters: "ref(qr/x/)" returns "Regexp"; however,
           dereferencing it is not well defined (you currently get the
           normalized version of the original pattern, but this may
           change).

           For example,

               $rex = qr/my.STRING/is;
               print $rex;                 # prints (?si-xm:my.STRING)
               s/$rex/foo/;

           is equivalent to

               s/my.STRING/foo/is;

           The result may be used as a subpattern in a match:

               $re = qr/$pattern/;
               $string =~ /foo${re}bar/;   # can be interpolated in other
                                           # patterns
               $string =~ $re;             # or used standalone
               $string =~ /$re/;           # or this way

           Since Perl may compile the pattern at the moment of execution
           of the "qr()" operator, using "qr()" may have speed advantages
           in some situations, notably if the result of "qr()" is used
           standalone:

               sub match {
                   my $patterns = shift;
                   my @compiled = map qr/$_/i, @$patterns;
                   grep {
                       my $success = 0;
                       foreach my $pat (@compiled) {
                           $success = 1, last if /$pat/;
                       }
                       $success;
                   } @_;
               }

           Precompilation of the pattern into an internal representation
           at the moment of "qr()" avoids the need to recompile the
           pattern every time a match "/$pat/" is attempted.  (Perl has
           many other internal optimizations, but none would be triggered
           in the above example if we did not use "qr()" operator.)

           Options (specified by the following modifiers) are:

               m   Treat string as multiple lines.
               s   Treat string as single line. (Make . match a newline)
               i   Do case-insensitive pattern matching.
               x   Use extended regular expressions.
               p   When matching preserve a copy of the matched string so
                   that ${^PREMATCH}, ${^MATCH}, ${^POSTMATCH} will be
                   defined (ignored starting in v5.20) as these are always
                   defined starting in that relese
               o   Compile pattern only once.
               a   ASCII-restrict: Use ASCII for \d, \s, \w; specifying two
                   a's further restricts things to that that no ASCII
                   character will match a non-ASCII one under /i.
               l   Use the current run-time locale's rules.
               u   Use Unicode rules.
               d   Use Unicode or native charset, as in 5.12 and earlier.
               n   Non-capture mode. Don't let () fill in $1, $2, etc...

           If a precompiled pattern is embedded in a larger pattern then
           the effect of "msixpluadn" will be propagated appropriately.
           The effect that the "/o" modifier has is not propagated, being
           restricted to those patterns explicitly using it.

           The last four modifiers listed above, added in Perl 5.14,
           control the character set rules, but "/a" is the only one you
           are likely to want to specify explicitly; the other three are
           selected automatically by various pragmas.

           See perlre for additional information on valid syntax for
           STRING, and for a detailed look at the semantics of regular
           expressions.  In particular, all modifiers except the largely
           obsolete "/o" are further explained in "Modifiers" in perlre.
           "/o" is described in the next section.

   "m/PATTERN/msixpodualngc"
   "/PATTERN/msixpodualngc"
           Searches a string for a pattern match, and in scalar context
           returns true if it succeeds, false if it fails.  If no string
           is specified via the "=~" or "!~" operator, the $_ string is
           searched.  (The string specified with "=~" need not be an
           lvalue--it may be the result of an expression evaluation, but
           remember the "=~" binds rather tightly.)  See also perlre.

           Options are as described in "qr//" above; in addition, the
           following match process modifiers are available:

            g  Match globally, i.e., find all occurrences.
            c  Do not reset search position on a failed match when /g is
               in effect.

           If "/" is the delimiter then the initial "m" is optional.  With
           the "m" you can use any pair of non-whitespace (ASCII)
           characters as delimiters.  This is particularly useful for
           matching path names that contain "/", to avoid LTS (leaning
           toothpick syndrome).  If "?" is the delimiter, then a match-
           only-once rule applies, described in "m?PATTERN?" below.  If
           "'" (single quote) is the delimiter, no interpolation is
           performed on the PATTERN.  When using a delimiter character
           valid in an identifier, whitespace is required after the "m".

           PATTERN may contain variables, which will be interpolated every
           time the pattern search is evaluated, except for when the
           delimiter is a single quote.  (Note that $(, $), and $| are not
           interpolated because they look like end-of-string tests.)  Perl
           will not recompile the pattern unless an interpolated variable
           that it contains changes.  You can force Perl to skip the test
           and never recompile by adding a "/o" (which stands for "once")
           after the trailing delimiter.  Once upon a time, Perl would
           recompile regular expressions unnecessarily, and this modifier
           was useful to tell it not to do so, in the interests of speed.
           But now, the only reasons to use "/o" are one of:

           1.  The variables are thousands of characters long and you know
               that they don't change, and you need to wring out the last
               little bit of speed by having Perl skip testing for that.
               (There is a maintenance penalty for doing this, as
               mentioning "/o" constitutes a promise that you won't change
               the variables in the pattern.  If you do change them, Perl
               won't even notice.)

           2.  you want the pattern to use the initial values of the
               variables regardless of whether they change or not.  (But
               there are saner ways of accomplishing this than using
               "/o".)

           3.  If the pattern contains embedded code, such as

                   use re 'eval';
                   $code = 'foo(?{ $x })';
                   /$code/

               then perl will recompile each time, even though the pattern
               string hasn't changed, to ensure that the current value of
               $x is seen each time.  Use "/o" if you want to avoid this.

           The bottom line is that using "/o" is almost never a good idea.

   The empty pattern "//"
           If the PATTERN evaluates to the empty string, the last
           successfully matched regular expression is used instead.  In
           this case, only the "g" and "c" flags on the empty pattern are
           honored; the other flags are taken from the original pattern.
           If no match has previously succeeded, this will (silently) act
           instead as a genuine empty pattern (which will always match).

           Note that it's possible to confuse Perl into thinking "//" (the
           empty regex) is really "//" (the defined-or operator).  Perl is
           usually pretty good about this, but some pathological cases
           might trigger this, such as "$x///" (is that "($x)/(//)" or
           "$x///"?) and "print$fh//" ("print$fh(//" or
           "print($fh//"?).  In all of these examples, Perl will assume
           you meant defined-or.  If you meant the empty regex, just use
           parentheses or spaces to disambiguate, or even prefix the empty
           regex with an "m" (so "//" becomes "m//").

   Matching in list context
           If the "/g" option is not used, "m//" in list context returns a
           list consisting of the subexpressions matched by the
           parentheses in the pattern, that is, ($1, $2, $3...)  (Note
           that here $1 etc. are also set).  When there are no parentheses
           in the pattern, the return value is the list "(1)" for success.
           With or without parentheses, an empty list is returned upon
           failure.

           Examples:

            open(TTY, "+</dev/tty")
               || die "can't access /dev/tty: $!";

            <TTY> =~ /^y/i && foo();       # do foo if desired

            if (/Version: *([0-9.]*)/) { $version = $1; }

            next if m#^/usr/spool/uucp#;

            # poor man's grep
            $arg = shift;
            while (<>) {
               print if /$arg/o; # compile only once (no longer needed!)
            }

            if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))

           This last example splits $foo into the first two words and the
           remainder of the line, and assigns those three fields to $F1,
           $F2, and $Etc.  The conditional is true if any variables were
           assigned; that is, if the pattern matched.

           The "/g" modifier specifies global pattern matching--that is,
           matching as many times as possible within the string.  How it
           behaves depends on the context.  In list context, it returns a
           list of the substrings matched by any capturing parentheses in
           the regular expression.  If there are no parentheses, it
           returns a list of all the matched strings, as if there were
           parentheses around the whole pattern.

           In scalar context, each execution of "m//g" finds the next
           match, returning true if it matches, and false if there is no
           further match.  The position after the last match can be read
           or set using the "pos()" function; see "pos" in perlfunc.  A
           failed match normally resets the search position to the
           beginning of the string, but you can avoid that by adding the
           "/c" modifier (for example, "m//gc").  Modifying the target
           string also resets the search position.

   "\G assertion"
           You can intermix "m//g" matches with "m/\G.../g", where "\G" is
           a zero-width assertion that matches the exact position where
           the previous "m//g", if any, left off.  Without the "/g"
           modifier, the "\G" assertion still anchors at "pos()" as it was
           at the start of the operation (see "pos" in perlfunc), but the
           match is of course only attempted once.  Using "\G" without
           "/g" on a target string that has not previously had a "/g"
           match applied to it is the same as using the "\A" assertion to
           match the beginning of the string.  Note also that, currently,
           "\G" is only properly supported when anchored at the very
           beginning of the pattern.

           Examples:

               # list context
               ($one,$five,$fifteen) = (`uptime` =~ /(\d+\.\d+)/g);

               # scalar context
               local $/ = "";
               while ($paragraph = <>) {
                   while ($paragraph =~ /\p{Ll}['")]*[.!?]+['")]*\s/g) {
                       $sentences++;
                   }
               }
               say $sentences;

           Here's another way to check for sentences in a paragraph:

            my $sentence_rx = qr{
               (?: (?<= ^ ) | (?<= \s ) )  # after start-of-string or
                                           # whitespace
               \p{Lu}                      # capital letter
               .*?                         # a bunch of anything
               (?<= \S )                   # that ends in non-
                                           # whitespace
               (?<! 	 [DMS]r  )           # but isn't a common abbr.
               (?<! 	 Mrs )
               (?<! 	 Sra )
               (?<! 	 St  )
               [.?!]                       # followed by a sentence
                                           # ender
               (?= $ | \s )                # in front of end-of-string
                                           # or whitespace
            }sx;
            local $/ = "";
            while (my $paragraph = <>) {
               say "NEW PARAGRAPH";
               my $count = 0;
               while ($paragraph =~ /($sentence_rx)/g) {
                   printf "\tgot sentence %d: <%s>\n", ++$count, $1;
               }
            }

           Here's how to use "m//gc" with "\G":

               $_ = "ppooqppqq";
               while ($i++ < 2) {
                   print "1: '";
                   print $1 while /(o)/gc; print "', pos=", pos, "\n";
                   print "2: '";
                   print $1 if /\G(q)/gc;  print "', pos=", pos, "\n";
                   print "3: '";
                   print $1 while /(p)/gc; print "', pos=", pos, "\n";
               }
               print "Final: '$1', pos=",pos,"\n" if /\G(.)/;

           The last example should print:

               1: 'oo', pos=4
               2: 'q', pos=5
               3: 'pp', pos=7
               1: '', pos=7
               2: 'q', pos=8
               3: '', pos=8
               Final: 'q', pos=8

           Notice that the final match matched "q" instead of "p", which a
           match without the "\G" anchor would have done.  Also note that
           the final match did not update "pos".  "pos" is only updated on
           a "/g" match.  If the final match did indeed match "p", it's a
           good bet that you're running a very old (pre-5.6.0) version of
           Perl.

           A useful idiom for "lex"-like scanners is "/\G.../gc".  You can
           combine several regexps like this to process a string part-by-
           part, doing different actions depending on which regexp
           matched.  Each regexp tries to match where the previous one
           leaves off.

            $_ = <<'EOL';
               $url = URI::URL->new( "http://example.com/" );
               die if $url eq "xXx";
            EOL

            LOOP: {
                print(" digits"),       redo LOOP if /\G\d+	[,.;]?\s*/gc;
                print(" lowercase"),    redo LOOP
                                               if /\G\p{Ll}+	[,.;]?\s*/gc;
                print(" UPPERCASE"),    redo LOOP
                                               if /\G\p{Lu}+	[,.;]?\s*/gc;
                print(" Capitalized"),  redo LOOP
                                         if /\G\p{Lu}\p{Ll}+	[,.;]?\s*/gc;
                print(" MiXeD"),        redo LOOP if /\G\pL+	[,.;]?\s*/gc;
                print(" alphanumeric"), redo LOOP
                                       if /\G[\p{Alpha}\pN]+	[,.;]?\s*/gc;
                print(" line-noise"),   redo LOOP if /\G\W+/gc;
                print ". That's all!\n";
            }

           Here is the output (split into several lines):

            line-noise lowercase line-noise UPPERCASE line-noise UPPERCASE
            line-noise lowercase line-noise lowercase line-noise lowercase
            lowercase line-noise lowercase lowercase line-noise lowercase
            lowercase line-noise MiXeD line-noise. That's all!

   "m?PATTERN?msixpodualngc"
   "?PATTERN?msixpodualngc"
           This is just like the "m/PATTERN/" search, except that it
           matches only once between calls to the "reset()" operator.
           This is a useful optimization when you want to see only the
           first occurrence of something in each file of a set of files,
           for instance.  Only "m??"  patterns local to the current
           package are reset.

               while (<>) {
                   if (m?^$?) {
                                       # blank line between header and body
                   }
               } continue {
                   reset if eof;       # clear m?? status for next file
               }

           Another example switched the first "latin1" encoding it finds
           to "utf8" in a pod file:

               s//utf8/ if m? ^ =encoding \h+ \K latin1 ?x;

           The match-once behavior is controlled by the match delimiter
           being "?"; with any other delimiter this is the normal "m//"
           operator.

           In the past, the leading "m" in "m?PATTERN?" was optional, but
           omitting it would produce a deprecation warning.  As of
           v5.22.0, omitting it produces a syntax error.  If you encounter
           this construct in older code, you can just add "m".

   "s/PATTERN/REPLACEMENT/msixpodualngcer"
           Searches a string for a pattern, and if found, replaces that
           pattern with the replacement text and returns the number of
           substitutions made.  Otherwise it returns false (specifically,
           the empty string).

           If the "/r" (non-destructive) option is used then it runs the
           substitution on a copy of the string and instead of returning
           the number of substitutions, it returns the copy whether or not
           a substitution occurred.  The original string is never changed
           when "/r" is used.  The copy will always be a plain string,
           even if the input is an object or a tied variable.

           If no string is specified via the "=~" or "!~" operator, the $_
           variable is searched and modified.  Unless the "/r" option is
           used, the string specified must be a scalar variable, an array
           element, a hash element, or an assignment to one of those; that
           is, some sort of scalar lvalue.

           If the delimiter chosen is a single quote, no interpolation is
           done on either the PATTERN or the REPLACEMENT.  Otherwise, if
           the PATTERN contains a "$" that looks like a variable rather
           than an end-of-string test, the variable will be interpolated
           into the pattern at run-time.  If you want the pattern compiled
           only once the first time the variable is interpolated, use the
           "/o" option.  If the pattern evaluates to the empty string, the
           last successfully executed regular expression is used instead.
           See perlre for further explanation on these.

           Options are as with "m//" with the addition of the following
           replacement specific options:

               e   Evaluate the right side as an expression.
               ee  Evaluate the right side as a string then eval the
                   result.
               r   Return substitution and leave the original string
                   untouched.

           Any non-whitespace delimiter may replace the slashes.  Add
           space after the "s" when using a character allowed in
           identifiers.  If single quotes are used, no interpretation is
           done on the replacement string (the "/e" modifier overrides
           this, however).  Note that Perl treats backticks as normal
           delimiters; the replacement text is not evaluated as a command.
           If the PATTERN is delimited by bracketing quotes, the
           REPLACEMENT has its own pair of quotes, which may or may not be
           bracketing quotes, for example, "s(foo)(bar)" or "s<foo>/bar/".
           A "/e" will cause the replacement portion to be treated as a
           full-fledged Perl expression and evaluated right then and
           there.  It is, however, syntax checked at compile-time.  A
           second "e" modifier will cause the replacement portion to be
           "eval"ed before being run as a Perl expression.

           Examples:

               s/	green	/mauve/g;              # don't change wintergreen

               $path =~ s|/usr/bin|/usr/local/bin|;

               s/Login: $foo/Login: $bar/; # run-time pattern

               ($foo = $bar) =~ s/this/that/;      # copy first, then
                                                   # change
               ($foo = "$bar") =~ s/this/that/;    # convert to string,
                                                   # copy, then change
               $foo = $bar =~ s/this/that/r;       # Same as above using /r
               $foo = $bar =~ s/this/that/r
                           =~ s/that/the other/r;  # Chained substitutes
                                                   # using /r
               @foo = map { s/this/that/r } @bar   # /r is very useful in
                                                   # maps

               $count = ($paragraph =~ s/Mister	/Mr./g);  # get change-cnt

               $_ = 'abc123xyz';
               s/\d+/$&*2/e;               # yields 'abc246xyz'
               s/\d+/sprintf("%5d",$&)/e;  # yields 'abc  246xyz'
               s/\w/$& x 2/eg;             # yields 'aabbcc  224466xxyyzz'

               s/%(.)/$percent{$1}/g;      # change percent escapes; no /e
               s/%(.)/$percent{$1} || $&/ge;       # expr now, so /e
               s/^=(\w+)/pod($1)/ge;       # use function call

               $_ = 'abc123xyz';
               $x = s/abc/def/r;           # $x is 'def123xyz' and
                                           # $_ remains 'abc123xyz'.

               # expand variables in $_, but dynamics only, using
               # symbolic dereferencing
               s/\$(\w+)/${$1}/g;

               # Add one to the value of any numbers in the string
               s/(\d+)/1 + $1/eg;

               # Titlecase words in the last 30 characters only
               substr($str, -30) =~ s/	(\p{Alpha}+)	/\u\L$1/g;

               # This will expand any embedded scalar variable
               # (including lexicals) in $_ : First $1 is interpolated
               # to the variable name, and then evaluated
               s/(\$\w+)/$1/eeg;

               # Delete (most) C comments.
               $program =~ s {
                   /\*     # Match the opening delimiter.
                   .*?     # Match a minimal number of characters.
                   \*/     # Match the closing delimiter.
               } []gsx;

               s/^\s*(.*?)\s*$/$1/;        # trim whitespace in $_,
                                           # expensively

               for ($variable) {           # trim whitespace in $variable,
                                           # cheap
                   s/^\s+//;
                   s/\s+$//;
               }

               s/([^ ]*) *([^ ]*)/$2 $1/;  # reverse 1st two fields

           Note the use of "$" instead of "\" in the last example.  Unlike
           sed, we use the \<digit> form only in the left hand side.
           Anywhere else it's $<digit>.

           Occasionally, you can't use just a "/g" to get all the changes
           to occur that you might want.  Here are two common cases:

               # put commas in the right places in an integer
               1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

               # expand tabs to 8-column spacing
               1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;

   Quote-Like Operators
   "q/STRING/"
   'STRING'
       A single-quoted, literal string.  A backslash represents a
       backslash unless followed by the delimiter or another backslash, in
       which case the delimiter or backslash is interpolated.

           $foo = q!I said, "You said, 'She said it.'"!;
           $bar = q('This is it.');
           $baz = '\n';                # a two-character string

   "qq/STRING/"
   "STRING"
       A double-quoted, interpolated string.

           $_ .= qq
            (*** The previous line contains the naughty word "$1".\n)
                       if /	(tcl|java|python)	/i;      # :-)
           $baz = "\n";                # a one-character string

   "qx/STRING/"
   "`STRING`"
       A string which is (possibly) interpolated and then executed as a
       system command with /bin/sh or its equivalent.  Shell wildcards,
       pipes, and redirections will be honored.  The collected standard
       output of the command is returned; standard error is unaffected.
       In scalar context, it comes back as a single (potentially multi-
       line) string, or "undef" if the command failed.  In list context,
       returns a list of lines (however you've defined lines with $/ or
       $INPUT_RECORD_SEPARATOR), or an empty list if the command failed.

       Because backticks do not affect standard error, use shell file
       descriptor syntax (assuming the shell supports this) if you care to
       address this.  To capture a command's STDERR and STDOUT together:

           $output = `cmd 2>&1`;

       To capture a command's STDOUT but discard its STDERR:

           $output = `cmd 2>/dev/null`;

       To capture a command's STDERR but discard its STDOUT (ordering is
       important here):

           $output = `cmd 2>&1 1>/dev/null`;

       To exchange a command's STDOUT and STDERR in order to capture the
       STDERR but leave its STDOUT to come out the old STDERR:

           $output = `cmd 3>&1 1>&2 2>&3 3>&-`;

       To read both a command's STDOUT and its STDERR separately, it's
       easiest to redirect them separately to files, and then read from
       those files when the program is done:

           system("program args 1>program.stdout 2>program.stderr");

       The STDIN filehandle used by the command is inherited from Perl's
       STDIN.  For example:

           open(SPLAT, "stuff")   || die "can't open stuff: $!";
           open(STDIN, "<&SPLAT") || die "can't dupe SPLAT: $!";
           print STDOUT `sort`;

       will print the sorted contents of the file named "stuff".

       Using single-quote as a delimiter protects the command from Perl's
       double-quote interpolation, passing it on to the shell instead:

           $perl_info  = qx(ps $$);            # that's Perl's $$
           $shell_info = qx'ps $$';            # that's the new shell's $$

       How that string gets evaluated is entirely subject to the command
       interpreter on your system.  On most platforms, you will have to
       protect shell metacharacters if you want them treated literally.
       This is in practice difficult to do, as it's unclear how to escape
       which characters.  See perlsec for a clean and safe example of a
       manual "fork()" and "exec()" to emulate backticks safely.

       On some platforms (notably DOS-like ones), the shell may not be
       capable of dealing with multiline commands, so putting newlines in
       the string may not get you what you want.  You may be able to
       evaluate multiple commands in a single line by separating them with
       the command separator character, if your shell supports that (for
       example, ";" on many Unix shells and "&" on the Windows NT "cmd"
       shell).

       Perl will attempt to flush all files opened for output before
       starting the child process, but this may not be supported on some
       platforms (see perlport).  To be safe, you may need to set $|
       ($AUTOFLUSH in "English") or call the "autoflush()" method of
       "IO::Handle" on any open handles.

       Beware that some command shells may place restrictions on the
       length of the command line.  You must ensure your strings don't
       exceed this limit after any necessary interpolations.  See the
       platform-specific release notes for more details about your
       particular environment.

       Using this operator can lead to programs that are difficult to
       port, because the shell commands called vary between systems, and
       may in fact not be present at all.  As one example, the "type"
       command under the POSIX shell is very different from the "type"
       command under DOS.  That doesn't mean you should go out of your way
       to avoid backticks when they're the right way to get something
       done.  Perl was made to be a glue language, and one of the things
       it glues together is commands.  Just understand what you're getting
       yourself into.

       Like "system", backticks put the child process exit code in $?.  If
       you'd like to manually inspect failure, you can check all possible
       failure modes by inspecting $? like this:

           if ($? == -1) {
               print "failed to execute: $!\n";
           }
           elsif ($? & 127) {
               printf "child died with signal %d, %s coredump\n",
                   ($? & 127),  ($? & 128) ? 'with' : 'without';
           }
           else {
               printf "child exited with value %d\n", $? >> 8;
           }

       See "I/O Operators" for more discussion.

   "qw/STRING/"
       Evaluates to a list of the words extracted out of STRING, using
       embedded whitespace as the word delimiters.  It can be understood
       as being roughly equivalent to:

           split(" ", q/STRING/);

       the differences being that it generates a real list at compile
       time, and in scalar context it returns the last element in the
       list.  So this expression:

           qw(foo bar baz)

       is semantically equivalent to the list:

           "foo", "bar", "baz"

       Some frequently seen examples:

           use POSIX qw( setlocale localeconv )
           @EXPORT = qw( foo bar baz );

       A common mistake is to try to separate the words with commas or to
       put comments into a multi-line "qw"-string.  For this reason, the
       "usewarnings" pragma and the -w switch (that is, the $^W variable)
       produces warnings if the STRING contains the "," or the "#"
       character.

   "tr/SEARCHLIST/REPLACEMENTLIST/cdsr"
   "y/SEARCHLIST/REPLACEMENTLIST/cdsr"
       Transliterates all occurrences of the characters found in the
       search list with the corresponding character in the replacement
       list.  It returns the number of characters replaced or deleted.  If
       no string is specified via the "=~" or "!~" operator, the $_ string
       is transliterated.

       If the "/r" (non-destructive) option is present, a new copy of the
       string is made and its characters transliterated, and this copy is
       returned no matter whether it was modified or not: the original
       string is always left unchanged.  The new copy is always a plain
       string, even if the input string is an object or a tied variable.

       Unless the "/r" option is used, the string specified with "=~" must
       be a scalar variable, an array element, a hash element, or an
       assignment to one of those; in other words, an lvalue.

       A character range may be specified with a hyphen, so "tr/A-J/0-9/"
       does the same replacement as "tr/ACEGIBDFHJ/0246813579/".  For sed
       devotees, "y" is provided as a synonym for "tr".  If the SEARCHLIST
       is delimited by bracketing quotes, the REPLACEMENTLIST has its own
       pair of quotes, which may or may not be bracketing quotes; for
       example, "tr[aeiouy][yuoiea]" or "tr(+\-*/)/ABCD/".

       Characters may be literals or any of the escape sequences accepted
       in double-quoted strings.  But there is no interpolation, so "$"
       and "@" are treated as literals.  A hyphen at the beginning or end,
       or preceded by a backslash is considered a literal.  Escape
       sequence details are in the table near the beginning of this
       section.

       Note that "tr" does not do regular expression character classes
       such as "\d" or "\pL".  The "tr" operator is not equivalent to the
       tr(1) utility.  If you want to map strings between lower/upper
       cases, see "lc" in perlfunc and "uc" in perlfunc, and in general
       consider using the "s" operator if you need regular expressions.
       The "\U", "\u", "\L", and "\l" string-interpolation escapes on the
       right side of a substitution operator will perform correct case-
       mappings, but "tr[a-z][A-Z]" will not (except sometimes on legacy
       7-bit data).

       Most ranges are unportable between character sets, but certain ones
       signal Perl to do special handling to make them portable.  There
       are two classes of portable ranges.  The first are any subsets of
       the ranges "A-Z", "a-z", and "0-9", when expressed as literal
       characters.

         tr/h-k/H-K/

       capitalizes the letters "h", "i", "j", and "k" and nothing else, no
       matter what the platform's character set is.  In contrast, all of

         tr/\x68-\x6B/\x48-\x4B/
         tr/h-\x6B/H-\x4B/
         tr/\x68-k/\x48-K/

       do the same capitalizations as the previous example when run on
       ASCII platforms, but something completely different on EBCDIC ones.

       The second class of portable ranges is invoked when one or both of
       the range's end points are expressed as "\N{...}"

        $string =~ tr/\N{U+20}-\N{U+7E}//d;

       removes from $string all the platform's characters which are
       equivalent to any of Unicode U+0020, U+0021, ... U+007D, U+007E.
       This is a portable range, and has the same effect on every platform
       it is run on.  It turns out that in this example, these are the
       ASCII printable characters.  So after this is run, $string has only
       controls and characters which have no ASCII equivalents.

       But, even for portable ranges, it is not generally obvious what is
       included without having to look things up.  A sound principle is to
       use only ranges that begin from and end at either ASCII alphabetics
       of equal case ("b-e", "b-E"), or digits ("1-4").  Anything else is
       unclear (and unportable unless "\N{...}" is used).  If in doubt,
       spell out the character sets in full.

       Options:

           c   Complement the SEARCHLIST.
           d   Delete found but unreplaced characters.
           s   Squash duplicate replaced characters.
           r   Return the modified string and leave the original string
               untouched.

       If the "/c" modifier is specified, the SEARCHLIST character set is
       complemented.  If the "/d" modifier is specified, any characters
       specified by SEARCHLIST not found in REPLACEMENTLIST are deleted.
       (Note that this is slightly more flexible than the behavior of some
       tr programs, which delete anything they find in the SEARCHLIST,
       period.)  If the "/s" modifier is specified, sequences of
       characters that were transliterated to the same character are
       squashed down to a single instance of the character.

       If the "/d" modifier is used, the REPLACEMENTLIST is always
       interpreted exactly as specified.  Otherwise, if the
       REPLACEMENTLIST is shorter than the SEARCHLIST, the final character
       is replicated till it is long enough.  If the REPLACEMENTLIST is
       empty, the SEARCHLIST is replicated.  This latter is useful for
       counting characters in a class or for squashing character sequences
       in a class.

       Examples:

           $ARGV[1] =~ tr/A-Z/a-z/;    # canonicalize to lower case ASCII

           $cnt = tr/*/*/;             # count the stars in $_

           $cnt = $sky =~ tr/*/*/;     # count the stars in $sky

           $cnt = tr/0-9//;            # count the digits in $_

           tr/a-zA-Z//s;               # bookkeeper -> bokeper

           ($HOST = $host) =~ tr/a-z/A-Z/;
            $HOST = $host  =~ tr/a-z/A-Z/r;   # same thing

           $HOST = $host =~ tr/a-z/A-Z/r    # chained with s///r
                         =~ s/:/ -p/r;

           tr/a-zA-Z/ /cs;             # change non-alphas to single space

           @stripped = map tr/a-zA-Z/ /csr, @original;
                                       # /r with map

           tr [\200-\377]
              [\000-\177];             # wickedly delete 8th bit

       If multiple transliterations are given for a character, only the
       first one is used:

           tr/AAA/XYZ/

       will transliterate any A to X.

       Because the transliteration table is built at compile time, neither
       the SEARCHLIST nor the REPLACEMENTLIST are subjected to double
       quote interpolation.  That means that if you want to use variables,
       you must use an "eval()":

           eval "tr/$oldlist/$newlist/";
           die $@ if $@;

           eval "tr/$oldlist/$newlist/, 1" or die $@;

   "<<EOF"
       A line-oriented form of quoting is based on the shell "here-
       document" syntax.  Following a "<<" you specify a string to
       terminate the quoted material, and all lines following the current
       line down to the terminating string are the value of the item.

       The terminating string may be either an identifier (a word), or
       some quoted text.  An unquoted identifier works like double quotes.
       There may not be a space between the "<<" and the identifier,
       unless the identifier is explicitly quoted.  (If you put a space it
       will be treated as a null identifier, which is valid, and matches
       the first empty line.)  The terminating string must appear by
       itself (unquoted and with no surrounding whitespace) on the
       terminating line.

       If the terminating string is quoted, the type of quotes used
       determine the treatment of the text.

       Double Quotes
           Double quotes indicate that the text will be interpolated using
           exactly the same rules as normal double quoted strings.

                  print <<EOF;
               The price is $Price.
               EOF

                  print << "EOF"; # same as above
               The price is $Price.
               EOF

       Single Quotes
           Single quotes indicate the text is to be treated literally with
           no interpolation of its content.  This is similar to single
           quoted strings except that backslashes have no special meaning,
           with "\\" being treated as two backslashes and not one as they
           would in every other quoting construct.

           Just as in the shell, a backslashed bareword following the "<<"
           means the same thing as a single-quoted string does:

                   $cost = <<'VISTA';  # hasta la ...
               That'll be $10 please, ma'am.
               VISTA

                   $cost = <<\VISTA;   # Same thing!
               That'll be $10 please, ma'am.
               VISTA

           This is the only form of quoting in perl where there is no need
           to worry about escaping content, something that code generators
           can and do make good use of.

       Backticks
           The content of the here doc is treated just as it would be if
           the string were embedded in backticks.  Thus the content is
           interpolated as though it were double quoted and then executed
           via the shell, with the results of the execution returned.

                  print << `EOC`; # execute command and get results
               echo hi there
               EOC

       It is possible to stack multiple here-docs in a row:

              print <<"foo", <<"bar"; # you can stack them
           I said foo.
           foo
           I said bar.
           bar

              myfunc(<< "THIS", 23, <<'THAT');
           Here's a line
           or two.
           THIS
           and here's another.
           THAT

       Just don't forget that you have to put a semicolon on the end to
       finish the statement, as Perl doesn't know you're not going to try
       to do this:

              print <<ABC
           179231
           ABC
              + 20;

       If you want to remove the line terminator from your here-docs, use
       "chomp()".

           chomp($string = <<'END');
           This is a string.
           END

       If you want your here-docs to be indented with the rest of the
       code, you'll need to remove leading whitespace from each line
       manually:

           ($quote = <<'FINIS') =~ s/^\s+//gm;
              The Road goes ever on and on,
              down from the door where it began.
           FINIS

       If you use a here-doc within a delimited construct, such as in
       "s///eg", the quoted material must still come on the line following
       the "<<FOO" marker, which means it may be inside the delimited
       construct:

           s/this/<<E . 'that'
           the other
           E
            . 'more '/eg;

       It works this way as of Perl 5.18.  Historically, it was
       inconsistent, and you would have to write

           s/this/<<E . 'that'
            . 'more '/eg;
           the other
           E

       outside of string evals.

       Additionally, quoting rules for the end-of-string identifier are
       unrelated to Perl's quoting rules.  "q()", "qq()", and the like are
       not supported in place of '' and "", and the only interpolation is
       for backslashing the quoting character:

           print << "abc\"def";
           testing...
           abc"def

       Finally, quoted strings cannot span multiple lines.  The general
       rule is that the identifier must be a string literal.  Stick with
       that, and you should be safe.

   Gory details of parsing quoted constructs
   When presented with something that might have several different
   interpretations, Perl uses the DWIM (that's "Do What I Mean") principle
   to pick the most probable interpretation.  This strategy is so
   successful that Perl programmers often do not suspect the ambivalence
   of what they write.  But from time to time, Perl's notions differ
   substantially from what the author honestly meant.

   This section hopes to clarify how Perl handles quoted constructs.
   Although the most common reason to learn this is to unravel
   labyrinthine regular expressions, because the initial steps of parsing
   are the same for all quoting operators, they are all discussed
   together.

   The most important Perl parsing rule is the first one discussed below:
   when processing a quoted construct, Perl first finds the end of that
   construct, then interprets its contents.  If you understand this rule,
   you may skip the rest of this section on the first reading.  The other
   rules are likely to contradict the user's expectations much less
   frequently than this first one.

   Some passes discussed below are performed concurrently, but because
   their results are the same, we consider them individually.  For
   different quoting constructs, Perl performs different numbers of
   passes, from one to four, but these passes are always performed in the
   same order.

   Finding the end
       The first pass is finding the end of the quoted construct.  This
       results in saving to a safe location a copy of the text (between
       the starting and ending delimiters), normalized as necessary to
       avoid needing to know what the original delimiters were.

       If the construct is a here-doc, the ending delimiter is a line that
       has a terminating string as the content.  Therefore "<<EOF" is
       terminated by "EOF" immediately followed by "\n" and starting from
       the first column of the terminating line.  When searching for the
       terminating line of a here-doc, nothing is skipped.  In other
       words, lines after the here-doc syntax are compared with the
       terminating string line by line.

       For the constructs except here-docs, single characters are used as
       starting and ending delimiters.  If the starting delimiter is an
       opening punctuation (that is "(", "[", "{", or "<"), the ending
       delimiter is the corresponding closing punctuation (that is ")",
       "]", "}", or ">").  If the starting delimiter is an unpaired
       character like "/" or a closing punctuation, the ending delimiter
       is the same as the starting delimiter.  Therefore a "/" terminates
       a "qq//" construct, while a "]" terminates both "qq[]" and "qq]]"
       constructs.

       When searching for single-character delimiters, escaped delimiters
       and "\\" are skipped.  For example, while searching for terminating
       "/", combinations of "\\" and "\/" are skipped.  If the delimiters
       are bracketing, nested pairs are also skipped.  For example, while
       searching for a closing "]" paired with the opening "[",
       combinations of "\\", "\]", and "\[" are all skipped, and nested
       "[" and "]" are skipped as well.  However, when backslashes are
       used as the delimiters (like "qq\\" and "tr\\\"), nothing is
       skipped.  During the search for the end, backslashes that escape
       delimiters or other backslashes are removed (exactly speaking, they
       are not copied to the safe location).

       For constructs with three-part delimiters ("s///", "y///", and
       "tr///"), the search is repeated once more.  If the first delimiter
       is not an opening punctuation, the three delimiters must be the
       same, such as "s!!!" and "tr)))", in which case the second
       delimiter terminates the left part and starts the right part at
       once.  If the left part is delimited by bracketing punctuation
       (that is "()", "[]", "{}", or "<>"), the right part needs another
       pair of delimiters such as "s(){}" and "tr[]//".  In these cases,
       whitespace and comments are allowed between the two parts, although
       the comment must follow at least one whitespace character;
       otherwise a character expected as the start of the comment may be
       regarded as the starting delimiter of the right part.

       During this search no attention is paid to the semantics of the
       construct.  Thus:

           "$hash{"$foo/$bar"}"

       or:

           m/
             bar       # NOT a comment, this slash / terminated m//!
            /x

       do not form legal quoted expressions.   The quoted part ends on the
       first """ and "/", and the rest happens to be a syntax error.
       Because the slash that terminated "m//" was followed by a "SPACE",
       the example above is not "m//x", but rather "m//" with no "/x"
       modifier.  So the embedded "#" is interpreted as a literal "#".

       Also no attention is paid to "\c\" (multichar control char syntax)
       during this search.  Thus the second "\" in "qq/\c\/" is
       interpreted as a part of "\/", and the following "/" is not
       recognized as a delimiter.  Instead, use "\034" or "\x1c" at the
       end of quoted constructs.

   Interpolation
       The next step is interpolation in the text obtained, which is now
       delimiter-independent.  There are multiple cases.

       "<<'EOF'"
           No interpolation is performed.  Note that the combination "\\"
           is left intact, since escaped delimiters are not available for
           here-docs.

       "m''", the pattern of "s'''"
           No interpolation is performed at this stage.  Any backslashed
           sequences including "\\" are treated at the stage to "parsing
           regular expressions".

       '', "q//", "tr'''", "y'''", the replacement of "s'''"
           The only interpolation is removal of "\" from pairs of "\\".
           Therefore "-" in "tr'''" and "y'''" is treated literally as a
           hyphen and no character range is available.  "\1" in the
           replacement of "s'''" does not work as $1.

       "tr///", "y///"
           No variable interpolation occurs.  String modifying
           combinations for case and quoting such as "\Q", "\U", and "\E"
           are not recognized.  The other escape sequences such as "\200"
           and "\t" and backslashed characters such as "\\" and "\-" are
           converted to appropriate literals.  The character "-" is
           treated specially and therefore "\-" is treated as a literal
           "-".

       "", "``", "qq//", "qx//", "<file*glob>", "<<"EOF""
           "\Q", "\U", "\u", "\L", "\l", "\F" (possibly paired with "\E")
           are converted to corresponding Perl constructs.  Thus,
           "$foo\Qbaz$bar" is converted to
           "$foo.(quotemeta("baz".$bar))" internally.  The other
           escape sequences such as "\200" and "\t" and backslashed
           characters such as "\\" and "\-" are replaced with appropriate
           expansions.

           Let it be stressed that whatever falls between "\Q" and "\E" is
           interpolated in the usual way.  Something like "\Q\\E" has no
           "\E" inside.  Instead, it has "\Q", "\\", and "E", so the
           result is the same as for "\\\\E".  As a general rule,
           backslashes between "\Q" and "\E" may lead to counterintuitive
           results.  So, "\Q\t\E" is converted to "quotemeta("\t")", which
           is the same as "\\\t" (since TAB is not alphanumeric).  Note
           also that:

             $str = '\t';
             return "\Q$str";

           may be closer to the conjectural intention of the writer of
           "\Q\t\E".

           Interpolated scalars and arrays are converted internally to the
           "join" and "." catenation operations.  Thus, "$fooXXX'@arr'"
           becomes:

             $foo . " XXX '" . (join $", @arr) . "'";

           All operations above are performed simultaneously, left to
           right.

           Because the result of "\QSTRING\E" has all metacharacters
           quoted, there is no way to insert a literal "$" or "@" inside a
           "\Q\E" pair.  If protected by "\", "$" will be quoted to become
           "\\\$"; if not, it is interpreted as the start of an
           interpolated scalar.

           Note also that the interpolation code needs to make a decision
           on where the interpolated scalar ends.  For instance, whether
           "a$x->{c}" really means:

             "a " . $x . " -> {c}";

           or:

             "a " . $x -> {c};

           Most of the time, the longest possible text that does not
           include spaces between components and which contains matching
           braces or brackets.  because the outcome may be determined by
           voting based on heuristic estimators, the result is not
           strictly predictable.  Fortunately, it's usually correct for
           ambiguous cases.

       the replacement of "s///"
           Processing of "\Q", "\U", "\u", "\L", "\l", "\F" and
           interpolation happens as with "qq//" constructs.

           It is at this step that "\1" is begrudgingly converted to $1 in
           the replacement text of "s///", in order to correct the
           incorrigible sed hackers who haven't picked up the saner idiom
           yet.  A warning is emitted if the "usewarnings" pragma or the
           -w command-line flag (that is, the $^W variable) was set.

       "RE" in "?RE?", "/RE/", "m/RE/", "s/RE/foo/",
           Processing of "\Q", "\U", "\u", "\L", "\l", "\F", "\E", and
           interpolation happens (almost) as with "qq//" constructs.

           Processing of "\N{...}" is also done here, and compiled into an
           intermediate form for the regex compiler.  (This is because, as
           mentioned below, the regex compilation may be done at execution
           time, and "\N{...}" is a compile-time construct.)

           However any other combinations of "\" followed by a character
           are not substituted but only skipped, in order to parse them as
           regular expressions at the following step.  As "\c" is skipped
           at this step, "@" of "\c@" in RE is possibly treated as an
           array symbol (for example @foo), even though the same text in
           "qq//" gives interpolation of "\c@".

           Code blocks such as "(?{BLOCK})" are handled by temporarily
           passing control back to the perl parser, in a similar way that
           an interpolated array subscript expression such as
           "foo$array[1+f("[xyz")]bar" would be.

           Moreover, inside "(?{BLOCK})", "(?#comment)", and a
           "#"-comment in a "/x"-regular expression, no processing is
           performed whatsoever.  This is the first step at which the
           presence of the "/x" modifier is relevant.

           Interpolation in patterns has several quirks: $|, $(, $), "@+"
           and "@-" are not interpolated, and constructs $var[SOMETHING]
           are voted (by several different estimators) to be either an
           array element or $var followed by an RE alternative.  This is
           where the notation "${arr[$bar]}" comes handy: "/${arr[0-9]}/"
           is interpreted as array element "-9", not as a regular
           expression from the variable $arr followed by a digit, which
           would be the interpretation of "/$arr[0-9]/".  Since voting
           among different estimators may occur, the result is not
           predictable.

           The lack of processing of "\\" creates specific restrictions on
           the post-processed text.  If the delimiter is "/", one cannot
           get the combination "\/" into the result of this step.  "/"
           will finish the regular expression, "\/" will be stripped to
           "/" on the previous step, and "\\/" will be left as is.
           Because "/" is equivalent to "\/" inside a regular expression,
           this does not matter unless the delimiter happens to be
           character special to the RE engine, such as in "s*foo*bar*",
           "m[foo]", or "?foo?"; or an alphanumeric char, as in:

             m m ^ a \s* b mmx;

           In the RE above, which is intentionally obfuscated for
           illustration, the delimiter is "m", the modifier is "mx", and
           after delimiter-removal the RE is the same as for
           "m/^a\s*b/mx".  There's more than one reason you're
           encouraged to restrict your delimiters to non-alphanumeric,
           non-whitespace choices.

       This step is the last one for all constructs except regular
       expressions, which are processed further.

   parsing regular expressions
       Previous steps were performed during the compilation of Perl code,
       but this one happens at run time, although it may be optimized to
       be calculated at compile time if appropriate.  After preprocessing
       described above, and possibly after evaluation if concatenation,
       joining, casing translation, or metaquoting are involved, the
       resulting string is passed to the RE engine for compilation.

       Whatever happens in the RE engine might be better discussed in
       perlre, but for the sake of continuity, we shall do so here.

       This is another step where the presence of the "/x" modifier is
       relevant.  The RE engine scans the string from left to right and
       converts it into a finite automaton.

       Backslashed characters are either replaced with corresponding
       literal strings (as with "\{"), or else they generate special nodes
       in the finite automaton (as with "	").  Characters special to the
       RE engine (such as "|") generate corresponding nodes or groups of
       nodes.  "(?#...)" comments are ignored.  All the rest is either
       converted to literal strings to match, or else is ignored (as is
       whitespace and "#"-style comments if "/x" is present).

       Parsing of the bracketed character class construct, "[...]", is
       rather different than the rule used for the rest of the pattern.
       The terminator of this construct is found using the same rules as
       for finding the terminator of a "{}"-delimited construct, the only
       exception being that "]" immediately following "[" is treated as
       though preceded by a backslash.

       The terminator of runtime "(?{...})" is found by temporarily
       switching control to the perl parser, which should stop at the
       point where the logically balancing terminating "}" is found.

       It is possible to inspect both the string given to RE engine and
       the resulting finite automaton.  See the arguments
       "debug"/"debugcolor" in the "usere" pragma, as well as Perl's -Dr
       command-line switch documented in "Command Switches" in perlrun.

   Optimization of regular expressions
       This step is listed for completeness only.  Since it does not
       change semantics, details of this step are not documented and are
       subject to change without notice.  This step is performed over the
       finite automaton that was generated during the previous pass.

       It is at this stage that "split()" silently optimizes "/^/" to mean
       "/^/m".

   I/O Operators
   There are several I/O operators you should know about.

   A string enclosed by backticks (grave accents) first undergoes double-
   quote interpolation.  It is then interpreted as an external command,
   and the output of that command is the value of the backtick string,
   like in a shell.  In scalar context, a single string consisting of all
   output is returned.  In list context, a list of values is returned, one
   per line of output.  (You can set $/ to use a different line
   terminator.)  The command is executed each time the pseudo-literal is
   evaluated.  The status value of the command is returned in $? (see
   perlvar for the interpretation of $?).  Unlike in csh, no translation
   is done on the return data--newlines remain newlines.  Unlike in any of
   the shells, single quotes do not hide variable names in the command
   from interpretation.  To pass a literal dollar-sign through to the
   shell you need to hide it with a backslash.  The generalized form of
   backticks is "qx//".  (Because backticks always undergo shell expansion
   as well, see perlsec for security concerns.)

   In scalar context, evaluating a filehandle in angle brackets yields the
   next line from that file (the newline, if any, included), or "undef" at
   end-of-file or on error.  When $/ is set to "undef" (sometimes known as
   file-slurp mode) and the file is empty, it returns '' the first time,
   followed by "undef" subsequently.

   Ordinarily you must assign the returned value to a variable, but there
   is one situation where an automatic assignment happens.  If and only if
   the input symbol is the only thing inside the conditional of a "while"
   statement (even if disguised as a "for(;;)" loop), the value is
   automatically assigned to the global variable $_, destroying whatever
   was there previously.  (This may seem like an odd thing to you, but
   you'll use the construct in almost every Perl script you write.)  The
   $_ variable is not implicitly localized.  You'll have to put a
   "local$_;" before the loop if you want that to happen.

   The following lines are equivalent:

       while (defined($_ = <STDIN>)) { print; }
       while ($_ = <STDIN>) { print; }
       while (<STDIN>) { print; }
       for (;<STDIN>;) { print; }
       print while defined($_ = <STDIN>);
       print while ($_ = <STDIN>);
       print while <STDIN>;

   This also behaves similarly, but assigns to a lexical variable instead
   of to $_:

       while (my $line = <STDIN>) { print $line }

   In these loop constructs, the assigned value (whether assignment is
   automatic or explicit) is then tested to see whether it is defined.
   The defined test avoids problems where the line has a string value that
   would be treated as false by Perl; for example a "" or a "0" with no
   trailing newline.  If you really mean for such values to terminate the
   loop, they should be tested for explicitly:

       while (($_ = <STDIN>) ne '0') { ... }
       while (<STDIN>) { last unless $_; ... }

   In other boolean contexts, "<FILEHANDLE>" without an explicit "defined"
   test or comparison elicits a warning if the "usewarnings" pragma or
   the -w command-line switch (the $^W variable) is in effect.

   The filehandles STDIN, STDOUT, and STDERR are predefined.  (The
   filehandles "stdin", "stdout", and "stderr" will also work except in
   packages, where they would be interpreted as local identifiers rather
   than global.)  Additional filehandles may be created with the "open()"
   function, amongst others.  See perlopentut and "open" in perlfunc for
   details on this.

   If a "<FILEHANDLE>" is used in a context that is looking for a list, a
   list comprising all input lines is returned, one line per list element.
   It's easy to grow to a rather large data space this way, so use with
   care.

   "<FILEHANDLE>"  may also be spelled "readline(*FILEHANDLE)".  See
   "readline" in perlfunc.

   The null filehandle "<>" is special: it can be used to emulate the
   behavior of sed and awk, and any other Unix filter program that takes a
   list of filenames, doing the same to each line of input from all of
   them.  Input from "<>" comes either from standard input, or from each
   file listed on the command line.  Here's how it works: the first time
   "<>" is evaluated, the @ARGV array is checked, and if it is empty,
   $ARGV[0] is set to "-", which when opened gives you standard input.
   The @ARGV array is then processed as a list of filenames.  The loop

       while (<>) {
           ...                     # code for each line
       }

   is equivalent to the following Perl-like pseudo code:

       unshift(@ARGV, '-') unless @ARGV;
       while ($ARGV = shift) {
           open(ARGV, $ARGV);
           while (<ARGV>) {
               ...         # code for each line
           }
       }

   except that it isn't so cumbersome to say, and will actually work.  It
   really does shift the @ARGV array and put the current filename into the
   $ARGV variable.  It also uses filehandle ARGV internally.  "<>" is just
   a synonym for "<ARGV>", which is magical.  (The pseudo code above
   doesn't work because it treats "<ARGV>" as non-magical.)

   Since the null filehandle uses the two argument form of "open" in
   perlfunc it interprets special characters, so if you have a script like
   this:

       while (<>) {
           print;
       }

   and call it with "perldangerous.pl'rm-rfv*|'", it actually opens a
   pipe, executes the "rm" command and reads "rm"'s output from that pipe.
   If you want all items in @ARGV to be interpreted as file names, you can
   use the module "ARGV::readonly" from CPAN, or use the double bracket:

       while (<<>>) {
           print;
       }

   Using double angle brackets inside of a while causes the open to use
   the three argument form (with the second argument being "<"), so all
   arguments in "ARGV" are treated as literal filenames (including "-").
   (Note that for convenience, if you use "<<>>" and if @ARGV is empty, it
   will still read from the standard input.)

   You can modify @ARGV before the first "<>" as long as the array ends up
   containing the list of filenames you really want.  Line numbers ($.)
   continue as though the input were one big happy file.  See the example
   in "eof" in perlfunc for how to reset line numbers on each file.

   If you want to set @ARGV to your own list of files, go right ahead.
   This sets @ARGV to all plain text files if no @ARGV was given:

       @ARGV = grep { -f && -T } glob('*') unless @ARGV;

   You can even set them to pipe commands.  For example, this
   automatically filters compressed arguments through gzip:

       @ARGV = map { /\.(gz|Z)$/ ? "gzip -dc < $_ |" : $_ } @ARGV;

   If you want to pass switches into your script, you can use one of the
   "Getopts" modules or put a loop on the front like this:

       while ($_ = $ARGV[0], /^-/) {
           shift;
           last if /^--$/;
           if (/^-D(.*)/) { $debug = $1 }
           if (/^-v/)     { $verbose++  }
           # ...           # other switches
       }

       while (<>) {
           # ...           # code for each line
       }

   The "<>" symbol will return "undef" for end-of-file only once.  If you
   call it again after this, it will assume you are processing another
   @ARGV list, and if you haven't set @ARGV, will read input from STDIN.

   If what the angle brackets contain is a simple scalar variable (for
   example, $foo), then that variable contains the name of the filehandle
   to input from, or its typeglob, or a reference to the same.  For
   example:

       $fh = \*STDIN;
       $line = <$fh>;

   If what's within the angle brackets is neither a filehandle nor a
   simple scalar variable containing a filehandle name, typeglob, or
   typeglob reference, it is interpreted as a filename pattern to be
   globbed, and either a list of filenames or the next filename in the
   list is returned, depending on context.  This distinction is determined
   on syntactic grounds alone.  That means "<$x>" is always a "readline()"
   from an indirect handle, but "<$hash{key}>" is always a "glob()".
   That's because $x is a simple scalar variable, but $hash{key} is
   not--it's a hash element.  Even "<$x >" (note the extra space) is
   treated as "glob("$x ")", not "readline($x)".

   One level of double-quote interpretation is done first, but you can't
   say "<$foo>" because that's an indirect filehandle as explained in the
   previous paragraph.  (In older versions of Perl, programmers would
   insert curly brackets to force interpretation as a filename glob:
   "<${foo}>".  These days, it's considered cleaner to call the internal
   function directly as "glob($foo)", which is probably the right way to
   have done it in the first place.)  For example:

       while (<*.c>) {
           chmod 0644, $_;
       }

   is roughly equivalent to:

       open(FOO, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'|");
       while (<FOO>) {
           chomp;
           chmod 0644, $_;
       }

   except that the globbing is actually done internally using the standard
   "File::Glob" extension.  Of course, the shortest way to do the above
   is:

       chmod 0644, <*.c>;

   A (file)glob evaluates its (embedded) argument only when it is starting
   a new list.  All values must be read before it will start over.  In
   list context, this isn't important because you automatically get them
   all anyway.  However, in scalar context the operator returns the next
   value each time it's called, or "undef" when the list has run out.  As
   with filehandle reads, an automatic "defined" is generated when the
   glob occurs in the test part of a "while", because legal glob returns
   (for example, a file called 0) would otherwise terminate the loop.
   Again, "undef" is returned only once.  So if you're expecting a single
   value from a glob, it is much better to say

       ($file) = <blurch*>;

   than

       $file = <blurch*>;

   because the latter will alternate between returning a filename and
   returning false.

   If you're trying to do variable interpolation, it's definitely better
   to use the "glob()" function, because the older notation can cause
   people to become confused with the indirect filehandle notation.

       @files = glob("$dir/*.[ch]");
       @files = glob($files[$i]);

   Constant Folding
   Like C, Perl does a certain amount of expression evaluation at compile
   time whenever it determines that all arguments to an operator are
   static and have no side effects.  In particular, string concatenation
   happens at compile time between literals that don't do variable
   substitution.  Backslash interpolation also happens at compile time.
   You can say

         'Now is the time for all'
       . "\n"
       .  'good men to come to.'

   and this all reduces to one string internally.  Likewise, if you say

       foreach $file (@filenames) {
           if (-s $file > 5 + 100 * 2**16) {  }
       }

   the compiler precomputes the number which that expression represents so
   that the interpreter won't have to.

   No-ops
   Perl doesn't officially have a no-op operator, but the bare constants 0
   and 1 are special-cased not to produce a warning in void context, so
   you can for example safely do

       1 while foo();

   Bitwise String Operators
   Bitstrings of any size may be manipulated by the bitwise operators ("~
   | & ^").

   If the operands to a binary bitwise op are strings of different sizes,
   | and ^ ops act as though the shorter operand had additional zero bits
   on the right, while the & op acts as though the longer operand were
   truncated to the length of the shorter.  The granularity for such
   extension or truncation is one or more bytes.

       # ASCII-based examples
       print "j p \n" ^ " a h";            # prints "JAPH\n"
       print "JA" | "  ph\n";              # prints "japh\n"
       print "japh\nJunk" & '_____';       # prints "JAPH\n";
       print 'p N$' ^ " E<H\n";            # prints "Perl\n";

   If you are intending to manipulate bitstrings, be certain that you're
   supplying bitstrings: If an operand is a number, that will imply a
   numeric bitwise operation.  You may explicitly show which type of
   operation you intend by using "" or "0+", as in the examples below.

       $foo =  150  |  105;        # yields 255  (0x96 | 0x69 is 0xFF)
       $foo = '150' |  105;        # yields 255
       $foo =  150  | '105';       # yields 255
       $foo = '150' | '105';       # yields string '155' (under ASCII)

       $baz = 0+$foo & 0+$bar;     # both ops explicitly numeric
       $biz = "$foo" ^ "$bar";     # both ops explicitly stringy

   This somewhat unpredictable behavior can be avoided with the
   experimental "bitwise" feature, new in Perl 5.22.  You can enable it
   via "usefeature'bitwise'".  By default, it will warn unless the
   "experimental::bitwise" warnings category has been disabled.
   ("useexperimental'bitwise'" will enable the feature and disable the
   warning.)  Under this feature, the four standard bitwise operators ("~
   | & ^") are always numeric.  Adding a dot after each operator ("~. |.
   &. ^.") forces it to treat its operands as strings:

       use experimental "bitwise";
       $foo =  150  |  105;        # yields 255  (0x96 | 0x69 is 0xFF)
       $foo = '150' |  105;        # yields 255
       $foo =  150  | '105';       # yields 255
       $foo = '150' | '105';       # yields 255
       $foo =  150  |. 105;        # yields string '155'
       $foo = '150' |. 105;        # yields string '155'
       $foo =  150  |.'105';       # yields string '155'
       $foo = '150' |.'105';       # yields string '155'

       $baz = $foo &  $bar;        # both operands numeric
       $biz = $foo ^. $bar;        # both operands stringy

   The assignment variants of these operators ("&= |= ^= &.= |.= ^.=")
   behave likewise under the feature.

   The behavior of these operators is problematic (and subject to change)
   if either or both of the strings are encoded in UTF-8 (see "Byte and
   Character Semantics" in perlunicode.

   See "vec" in perlfunc for information on how to manipulate individual
   bits in a bit vector.

   Integer Arithmetic
   By default, Perl assumes that it must do most of its arithmetic in
   floating point.  But by saying

       use integer;

   you may tell the compiler to use integer operations (see integer for a
   detailed explanation) from here to the end of the enclosing BLOCK.  An
   inner BLOCK may countermand this by saying

       no integer;

   which lasts until the end of that BLOCK.  Note that this doesn't mean
   everything is an integer, merely that Perl will use integer operations
   for arithmetic, comparison, and bitwise operators.  For example, even
   under "useinteger", if you take the sqrt(2), you'll still get
   1.4142135623731 or so.

   Used on numbers, the bitwise operators ("&" "|" "^" "~" "<<" ">>")
   always produce integral results.  (But see also "Bitwise String
   Operators".)  However, "useinteger" still has meaning for them.  By
   default, their results are interpreted as unsigned integers, but if
   "useinteger" is in effect, their results are interpreted as signed
   integers.  For example, "~0" usually evaluates to a large integral
   value.  However, "useinteger;~0" is "-1" on two's-complement
   machines.

   Floating-point Arithmetic
   While "useinteger" provides integer-only arithmetic, there is no
   analogous mechanism to provide automatic rounding or truncation to a
   certain number of decimal places.  For rounding to a certain number of
   digits, "sprintf()" or "printf()" is usually the easiest route.  See
   perlfaq4.

   Floating-point numbers are only approximations to what a mathematician
   would call real numbers.  There are infinitely more reals than floats,
   so some corners must be cut.  For example:

       printf "%.20g\n", 123456789123456789;
       #        produces 123456789123456784

   Testing for exact floating-point equality or inequality is not a good
   idea.  Here's a (relatively expensive) work-around to compare whether
   two floating-point numbers are equal to a particular number of decimal
   places.  See Knuth, volume II, for a more robust treatment of this
   topic.

       sub fp_equal {
           my ($X, $Y, $POINTS) = @_;
           my ($tX, $tY);
           $tX = sprintf("%.${POINTS}g", $X);
           $tY = sprintf("%.${POINTS}g", $Y);
           return $tX eq $tY;
       }

   The POSIX module (part of the standard perl distribution) implements
   "ceil()", "floor()", and other mathematical and trigonometric
   functions.  The "Math::Complex" module (part of the standard perl
   distribution) defines mathematical functions that work on both the
   reals and the imaginary numbers.  "Math::Complex" is not as efficient
   as POSIX, but POSIX can't work with complex numbers.

   Rounding in financial applications can have serious implications, and
   the rounding method used should be specified precisely.  In these
   cases, it probably pays not to trust whichever system rounding is being
   used by Perl, but to instead implement the rounding function you need
   yourself.

   Bigger Numbers
   The standard "Math::BigInt", "Math::BigRat", and "Math::BigFloat"
   modules, along with the "bignum", "bigint", and "bigrat" pragmas,
   provide variable-precision arithmetic and overloaded operators,
   although they're currently pretty slow.  At the cost of some space and
   considerable speed, they avoid the normal pitfalls associated with
   limited-precision representations.

           use 5.010;
           use bigint;  # easy interface to Math::BigInt
           $x = 123456789123456789;
           say $x * $x;
       +15241578780673678515622620750190521

   Or with rationals:

           use 5.010;
           use bigrat;
           $x = 3/22;
           $y = 4/6;
           say "x/y is ", $x/$y;
           say "x*y is ", $x*$y;
           x/y is 9/44
           x*y is 1/11

   Several modules let you calculate with unlimited or fixed precision
   (bound only by memory and CPU time).  There are also some non-standard
   modules that provide faster implementations via external C libraries.

   Here is a short, but incomplete summary:

     Math::String           treat string sequences like numbers
     Math::FixedPrecision   calculate with a fixed precision
     Math::Currency         for currency calculations
     Bit::Vector            manipulate bit vectors fast (uses C)
     Math::BigIntFast       Bit::Vector wrapper for big numbers
     Math::Pari             provides access to the Pari C library
     Math::Cephes           uses the external Cephes C library (no
                            big numbers)
     Math::Cephes::Fraction fractions via the Cephes library
     Math::GMP              another one using an external C library
     Math::GMPz             an alternative interface to libgmp's big ints
     Math::GMPq             an interface to libgmp's fraction numbers
     Math::GMPf             an interface to libgmp's floating point numbers

   Choose wisely.





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