perlrecharclass(1)
NAME
perlrecharclass - Perl Regular Expression Character Classes
DESCRIPTION
The top level documentation about Perl regular expressions is found in
perlre.
This manual page discusses the syntax and use of character classes in
Perl regular expressions.
A character class is a way of denoting a set of characters in such a
way that one character of the set is matched. It's important to
remember that: matching a character class consumes exactly one
character in the source string. (The source string is the string the
regular expression is matched against.)
There are three types of character classes in Perl regular expressions:
the dot, backslash sequences, and the form enclosed in square brackets.
Keep in mind, though, that often the term "character class" is used to
mean just the bracketed form. Certainly, most Perl documentation does
that.
The dot
The dot (or period), "." is probably the most used, and certainly the
most well-known character class. By default, a dot matches any
character, except for the newline. That default can be changed to add
matching the newline by using the single line modifier: either for the
entire regular expression with the "/s" modifier, or locally with
"(?s)". (The "\N" backslash sequence, described below, matches any
character except newline without regard to the single line modifier.)
Here are some examples:
"a" =~ /./ # Match
"." =~ /./ # Match
"" =~ /./ # No match (dot has to match a character)
"\n" =~ /./ # No match (dot does not match a newline)
"\n" =~ /./s # Match (global 'single line' modifier)
"\n" =~ /(?s:.)/ # Match (local 'single line' modifier)
"ab" =~ /^.$/ # No match (dot matches one character)
Backslash sequences
A backslash sequence is a sequence of characters, the first one of
which is a backslash. Perl ascribes special meaning to many such
sequences, and some of these are character classes. That is, they
match a single character each, provided that the character belongs to
the specific set of characters defined by the sequence.
Here's a list of the backslash sequences that are character classes.
They are discussed in more detail below. (For the backslash sequences
that aren't character classes, see perlrebackslash.)
\d Match a decimal digit character.
\D Match a non-decimal-digit character.
\w Match a "word" character.
\W Match a non-"word" character.
\s Match a whitespace character.
\S Match a non-whitespace character.
\h Match a horizontal whitespace character.
\H Match a character that isn't horizontal whitespace.
\v Match a vertical whitespace character.
\V Match a character that isn't vertical whitespace.
\N Match a character that isn't a newline.
\pP, \p{Prop} Match a character that has the given Unicode property.
\PP, \P{Prop} Match a character that doesn't have the Unicode property
\N
"\N", available starting in v5.12, like the dot, matches any character
that is not a newline. The difference is that "\N" is not influenced by
the single line regular expression modifier (see "The dot" above).
Note that the form "\N{...}" may mean something completely different.
When the "{...}" is a quantifier, it means to match a non-newline
character that many times. For example, "\N{3}" means to match 3 non-
newlines; "\N{5,}" means to match 5 or more non-newlines. But if
"{...}" is not a legal quantifier, it is presumed to be a named
character. See charnames for those. For example, none of "\N{COLON}",
"\N{4F}", and "\N{F4}" contain legal quantifiers, so Perl will try to
find characters whose names are respectively "COLON", "4F", and "F4".
Digits
"\d" matches a single character considered to be a decimal digit. If
the "/a" regular expression modifier is in effect, it matches [0-9].
Otherwise, it matches anything that is matched by "\p{Digit}", which
includes [0-9]. (An unlikely possible exception is that under locale
matching rules, the current locale might not have "[0-9]" matched by
"\d", and/or might match other characters whose code point is less than
256. The only such locale definitions that are legal would be to match
"[0-9]" plus another set of 10 consecutive digit characters; anything
else would be in violation of the C language standard, but Perl doesn't
currently assume anything in regard to this.)
What this means is that unless the "/a" modifier is in effect "\d" not
only matches the digits '0' - '9', but also Arabic, Devanagari, and
digits from other languages. This may cause some confusion, and some
security issues.
Some digits that "\d" matches look like some of the [0-9] ones, but
have different values. For example, BENGALI DIGIT FOUR (U+09EA) looks
very much like an ASCII DIGIT EIGHT (U+0038). An application that is
expecting only the ASCII digits might be misled, or if the match is
"\d+", the matched string might contain a mixture of digits from
different writing systems that look like they signify a number
different than they actually do. "num()" in Unicode::UCD can be used
to safely calculate the value, returning "undef" if the input string
contains such a mixture.
What "\p{Digit}" means (and hence "\d" except under the "/a" modifier)
is "\p{General_Category=Decimal_Number}", or synonymously,
"\p{General_Category=Digit}". Starting with Unicode version 4.1, this
is the same set of characters matched by "\p{Numeric_Type=Decimal}".
But Unicode also has a different property with a similar name,
"\p{Numeric_Type=Digit}", which matches a completely different set of
characters. These characters are things such as "CIRCLED DIGIT ONE" or
subscripts, or are from writing systems that lack all ten digits.
The design intent is for "\d" to exactly match the set of characters
that can safely be used with "normal" big-endian positional decimal
syntax, where, for example 123 means one 'hundred', plus two 'tens',
plus three 'ones'. This positional notation does not necessarily apply
to characters that match the other type of "digit",
"\p{Numeric_Type=Digit}", and so "\d" doesn't match them.
The Tamil digits (U+0BE6 - U+0BEF) can also legally be used in old-
style Tamil numbers in which they would appear no more than one in a
row, separated by characters that mean "times 10", "times 100", etc.
(See <http://www.unicode.org/notes/tn21>.)
Any character not matched by "\d" is matched by "\D".
Word characters
A "\w" matches a single alphanumeric character (an alphabetic
character, or a decimal digit); or a connecting punctuation character,
such as an underscore ("_"); or a "mark" character (like some sort of
accent) that attaches to one of those. It does not match a whole word.
To match a whole word, use "\w+". This isn't the same thing as
matching an English word, but in the ASCII range it is the same as a
string of Perl-identifier characters.
If the "/a" modifier is in effect ...
"\w" matches the 63 characters [a-zA-Z0-9_].
otherwise ...
For code points above 255 ...
"\w" matches the same as "\p{Word}" matches in this range.
That is, it matches Thai letters, Greek letters, etc. This
includes connector punctuation (like the underscore) which
connect two words together, or diacritics, such as a "COMBINING
TILDE" and the modifier letters, which are generally used to
add auxiliary markings to letters.
For code points below 256 ...
if locale rules are in effect ...
"\w" matches the platform's native underscore character
plus whatever the locale considers to be alphanumeric.
if Unicode rules are in effect ...
"\w" matches exactly what "\p{Word}" matches.
otherwise ...
"\w" matches [a-zA-Z0-9_].
Which rules apply are determined as described in "Which character set
modifier is in effect?" in perlre.
There are a number of security issues with the full Unicode list of
word characters. See <http://unicode.org/reports/tr36>.
Also, for a somewhat finer-grained set of characters that are in
programming language identifiers beyond the ASCII range, you may wish
to instead use the more customized "Unicode Properties",
"\p{ID_Start}", "\p{ID_Continue}", "\p{XID_Start}", and
"\p{XID_Continue}". See <http://unicode.org/reports/tr31>.
Any character not matched by "\w" is matched by "\W".
Whitespace
"\s" matches any single character considered whitespace.
If the "/a" modifier is in effect ...
In all Perl versions, "\s" matches the 5 characters [\t\n\f\r ];
that is, the horizontal tab, the newline, the form feed, the
carriage return, and the space. Starting in Perl v5.18, it also
matches the vertical tab, "\cK". See note "[1]" below for a
discussion of this.
otherwise ...
For code points above 255 ...
"\s" matches exactly the code points above 255 shown with an
"s" column in the table below.
For code points below 256 ...
if locale rules are in effect ...
"\s" matches whatever the locale considers to be
whitespace.
if Unicode rules are in effect ...
"\s" matches exactly the characters shown with an "s"
column in the table below.
otherwise ...
"\s" matches [\t\n\f\r ] and, starting in Perl v5.18, the
vertical tab, "\cK". (See note "[1]" below for a
discussion of this.) Note that this list doesn't include
the non-breaking space.
Which rules apply are determined as described in "Which character set
modifier is in effect?" in perlre.
Any character not matched by "\s" is matched by "\S".
"\h" matches any character considered horizontal whitespace; this
includes the platform's space and tab characters and several others
listed in the table below. "\H" matches any character not considered
horizontal whitespace. They use the platform's native character set,
and do not consider any locale that may otherwise be in use.
"\v" matches any character considered vertical whitespace; this
includes the platform's carriage return and line feed characters
(newline) plus several other characters, all listed in the table below.
"\V" matches any character not considered vertical whitespace. They
use the platform's native character set, and do not consider any locale
that may otherwise be in use.
"\R" matches anything that can be considered a newline under Unicode
rules. It can match a multi-character sequence. It cannot be used
inside a bracketed character class; use "\v" instead (vertical
whitespace). It uses the platform's native character set, and does not
consider any locale that may otherwise be in use. Details are
discussed in perlrebackslash.
Note that unlike "\s" (and "\d" and "\w"), "\h" and "\v" always match
the same characters, without regard to other factors, such as the
active locale or whether the source string is in UTF-8 format.
One might think that "\s" is equivalent to "[\h\v]". This is indeed
true starting in Perl v5.18, but prior to that, the sole difference was
that the vertical tab ("\cK") was not matched by "\s".
The following table is a complete listing of characters matched by
"\s", "\h" and "\v" as of Unicode 6.3.
The first column gives the Unicode code point of the character (in hex
format), the second column gives the (Unicode) name. The third column
indicates by which class(es) the character is matched (assuming no
locale is in effect that changes the "\s" matching).
0x0009 CHARACTER TABULATION h s
0x000a LINE FEED (LF) vs
0x000b LINE TABULATION vs [1]
0x000c FORM FEED (FF) vs
0x000d CARRIAGE RETURN (CR) vs
0x0020 SPACE h s
0x0085 NEXT LINE (NEL) vs [2]
0x00a0 NO-BREAK SPACE h s [2]
0x1680 OGHAM SPACE MARK h s
0x2000 EN QUAD h s
0x2001 EM QUAD h s
0x2002 EN SPACE h s
0x2003 EM SPACE h s
0x2004 THREE-PER-EM SPACE h s
0x2005 FOUR-PER-EM SPACE h s
0x2006 SIX-PER-EM SPACE h s
0x2007 FIGURE SPACE h s
0x2008 PUNCTUATION SPACE h s
0x2009 THIN SPACE h s
0x200a HAIR SPACE h s
0x2028 LINE SEPARATOR vs
0x2029 PARAGRAPH SEPARATOR vs
0x202f NARROW NO-BREAK SPACE h s
0x205f MEDIUM MATHEMATICAL SPACE h s
0x3000 IDEOGRAPHIC SPACE h s
[1] Prior to Perl v5.18, "\s" did not match the vertical tab.
"[^\S\cK]" (obscurely) matches what "\s" traditionally did.
[2] NEXT LINE and NO-BREAK SPACE may or may not match "\s" depending on
the rules in effect. See the beginning of this section.
Unicode Properties
"\pP" and "\p{Prop}" are character classes to match characters that fit
given Unicode properties. One letter property names can be used in the
"\pP" form, with the property name following the "\p", otherwise,
braces are required. When using braces, there is a single form, which
is just the property name enclosed in the braces, and a compound form
which looks like "\p{name=value}", which means to match if the property
"name" for the character has that particular "value". For instance, a
match for a number can be written as "/\pN/" or as "/\p{Number}/", or
as "/\p{Number=True}/". Lowercase letters are matched by the property
Lowercase_Letter which has the short form Ll. They need the braces, so
are written as "/\p{Ll}/" or "/\p{Lowercase_Letter}/", or
"/\p{General_Category=Lowercase_Letter}/" (the underscores are
optional). "/\pLl/" is valid, but means something different. It
matches a two character string: a letter (Unicode property "\pL"),
followed by a lowercase "l".
If locale rules are not in effect, the use of a Unicode property will
force the regular expression into using Unicode rules, if it isn't
already.
Note that almost all properties are immune to case-insensitive
matching. That is, adding a "/i" regular expression modifier does not
change what they match. There are two sets that are affected. The
first set is "Uppercase_Letter", "Lowercase_Letter", and
"Titlecase_Letter", all of which match "Cased_Letter" under "/i"
matching. The second set is "Uppercase", "Lowercase", and "Titlecase",
all of which match "Cased" under "/i" matching. (The difference
between these sets is that some things, such as Roman numerals, come in
both upper and lower case, so they are "Cased", but aren't considered
to be letters, so they aren't "Cased_Letter"s. They're actually
"Letter_Number"s.) This set also includes its subsets "PosixUpper" and
"PosixLower", both of which under "/i" match "PosixAlpha".
For more details on Unicode properties, see "Unicode Character
Properties" in perlunicode; for a complete list of possible properties,
see "Properties accessible through \p{} and \P{}" in perluniprops,
which notes all forms that have "/i" differences. It is also possible
to define your own properties. This is discussed in "User-Defined
Character Properties" in perlunicode.
Unicode properties are defined (surprise!) only on Unicode code points.
Starting in v5.20, when matching against "\p" and "\P", Perl treats
non-Unicode code points (those above the legal Unicode maximum of
0x10FFFF) as if they were typical unassigned Unicode code points.
Prior to v5.20, Perl raised a warning and made all matches fail on non-
Unicode code points. This could be somewhat surprising:
chr(0x110000) =~ \p{ASCII_Hex_Digit=True} # Fails on Perls < v5.20.
chr(0x110000) =~ \p{ASCII_Hex_Digit=False} # Also fails on Perls
# < v5.20
Even though these two matches might be thought of as complements, until
v5.20 they were so only on Unicode code points.
Examples
"a" =~ /\w/ # Match, "a" is a 'word' character.
"7" =~ /\w/ # Match, "7" is a 'word' character as well.
"a" =~ /\d/ # No match, "a" isn't a digit.
"7" =~ /\d/ # Match, "7" is a digit.
" " =~ /\s/ # Match, a space is whitespace.
"a" =~ /\D/ # Match, "a" is a non-digit.
"7" =~ /\D/ # No match, "7" is not a non-digit.
" " =~ /\S/ # No match, a space is not non-whitespace.
" " =~ /\h/ # Match, space is horizontal whitespace.
" " =~ /\v/ # No match, space is not vertical whitespace.
"\r" =~ /\v/ # Match, a return is vertical whitespace.
"a" =~ /\pL/ # Match, "a" is a letter.
"a" =~ /\p{Lu}/ # No match, /\p{Lu}/ matches upper case letters.
"\x{0e0b}" =~ /\p{Thai}/ # Match, \x{0e0b} is the character
# 'THAI CHARACTER SO SO', and that's in
# Thai Unicode class.
"a" =~ /\P{Lao}/ # Match, as "a" is not a Laotian character.
It is worth emphasizing that "\d", "\w", etc, match single characters,
not complete numbers or words. To match a number (that consists of
digits), use "\d+"; to match a word, use "\w+". But be aware of the
security considerations in doing so, as mentioned above.
Bracketed Character Classes
The third form of character class you can use in Perl regular
expressions is the bracketed character class. In its simplest form, it
lists the characters that may be matched, surrounded by square
brackets, like this: "[aeiou]". This matches one of "a", "e", "i", "o"
or "u". Like the other character classes, exactly one character is
matched.* To match a longer string consisting of characters mentioned
in the character class, follow the character class with a quantifier.
For instance, "[aeiou]+" matches one or more lowercase English vowels.
Repeating a character in a character class has no effect; it's
considered to be in the set only once.
Examples:
"e" =~ /[aeiou]/ # Match, as "e" is listed in the class.
"p" =~ /[aeiou]/ # No match, "p" is not listed in the class.
"ae" =~ /^[aeiou]$/ # No match, a character class only matches
# a single character.
"ae" =~ /^[aeiou]+$/ # Match, due to the quantifier.
-------
* There are two exceptions to a bracketed character class matching a
single character only. Each requires special handling by Perl to make
things work:
* When the class is to match caselessly under "/i" matching rules,
and a character that is explicitly mentioned inside the class
matches a multiple-character sequence caselessly under Unicode
rules, the class will also match that sequence. For example,
Unicode says that the letter "LATIN SMALL LETTER SHARP S" should
match the sequence "ss" under "/i" rules. Thus,
'ss' =~ /\A\N{LATIN SMALL LETTER SHARP S}\z/i # Matches
'ss' =~ /\A[aeioust\N{LATIN SMALL LETTER SHARP S}]\z/i # Matches
For this to happen, the class must not be inverted (see "Negation")
and the character must be explicitly specified, and not be part of
a multi-character range (not even as one of its endpoints).
("Character Ranges" will be explained shortly.) Therefore,
'ss' =~ /\A[\0-\x{ff}]\z/ui # Doesn't match
'ss' =~ /\A[\0-\N{LATIN SMALL LETTER SHARP S}]\z/ui # No match
'ss' =~ /\A[\xDF-\xDF]\z/ui # Matches on ASCII platforms, since
# \xDF is LATIN SMALL LETTER SHARP S,
# and the range is just a single
# element
Note that it isn't a good idea to specify these types of ranges
anyway.
* Some names known to "\N{...}" refer to a sequence of multiple
characters, instead of the usual single character. When one of
these is included in the class, the entire sequence is matched.
For example,
"\N{TAMIL LETTER KA}\N{TAMIL VOWEL SIGN AU}"
=~ / ^ [\N{TAMIL SYLLABLE KAU}] $ /x;
matches, because "\N{TAMIL SYLLABLE KAU}" is a named sequence
consisting of the two characters matched against. Like the other
instance where a bracketed class can match multiple characters, and
for similar reasons, the class must not be inverted, and the named
sequence may not appear in a range, even one where it is both
endpoints. If these happen, it is a fatal error if the character
class is within an extended "(?[...])" class; and only the first
code point is used (with a "regexp"-type warning raised) otherwise.
Special Characters Inside a Bracketed Character Class
Most characters that are meta characters in regular expressions (that
is, characters that carry a special meaning like ".", "*", or "(") lose
their special meaning and can be used inside a character class without
the need to escape them. For instance, "[()]" matches either an opening
parenthesis, or a closing parenthesis, and the parens inside the
character class don't group or capture.
Characters that may carry a special meaning inside a character class
are: "\", "^", "-", "[" and "]", and are discussed below. They can be
escaped with a backslash, although this is sometimes not needed, in
which case the backslash may be omitted.
The sequence " " is special inside a bracketed character class. While
outside the character class, " " is an assertion indicating a point
that does not have either two word characters or two non-word
characters on either side, inside a bracketed character class, " "
matches a backspace character.
The sequences "
", "\c", "\e", "\f", "\n", "\N{NAME}", "\N{U+hex
char}", "\r", "\t", and "\x" are also special and have the same
meanings as they do outside a bracketed character class.
Also, a backslash followed by two or three octal digits is considered
an octal number.
A "[" is not special inside a character class, unless it's the start of
a POSIX character class (see "POSIX Character Classes" below). It
normally does not need escaping.
A "]" is normally either the end of a POSIX character class (see "POSIX
Character Classes" below), or it signals the end of the bracketed
character class. If you want to include a "]" in the set of
characters, you must generally escape it.
However, if the "]" is the first (or the second if the first character
is a caret) character of a bracketed character class, it does not
denote the end of the class (as you cannot have an empty class) and is
considered part of the set of characters that can be matched without
escaping.
Examples:
"+" =~ /[+?*]/ # Match, "+" in a character class is not special.
"\cH" =~ /[ ]/ # Match, inside in a character class
# is equivalent to a backspace.
"]" =~ /[][]/ # Match, as the character class contains
# both [ and ].
"[]" =~ /[[]]/ # Match, the pattern contains a character class
# containing just [, and the character class is
# followed by a ].
Character Ranges
It is not uncommon to want to match a range of characters. Luckily,
instead of listing all characters in the range, one may use the hyphen
("-"). If inside a bracketed character class you have two characters
separated by a hyphen, it's treated as if all characters between the
two were in the class. For instance, "[0-9]" matches any ASCII digit,
and "[a-m]" matches any lowercase letter from the first half of the
ASCII alphabet.
Note that the two characters on either side of the hyphen are not
necessarily both letters or both digits. Any character is possible,
although not advisable. "['-?]" contains a range of characters, but
most people will not know which characters that means. Furthermore,
such ranges may lead to portability problems if the code has to run on
a platform that uses a different character set, such as EBCDIC.
If a hyphen in a character class cannot syntactically be part of a
range, for instance because it is the first or the last character of
the character class, or if it immediately follows a range, the hyphen
isn't special, and so is considered a character to be matched
literally. If you want a hyphen in your set of characters to be
matched and its position in the class is such that it could be
considered part of a range, you must escape that hyphen with a
backslash.
Examples:
[a-z] # Matches a character that is a lower case ASCII letter.
[a-fz] # Matches any letter between 'a' and 'f' (inclusive) or
# the letter 'z'.
[-z] # Matches either a hyphen ('-') or the letter 'z'.
[a-f-m] # Matches any letter between 'a' and 'f' (inclusive), the
# hyphen ('-'), or the letter 'm'.
['-?] # Matches any of the characters '()*+,-./0123456789:;<=>?
# (But not on an EBCDIC platform).
[\N{APOSTROPHE}-\N{QUESTION MARK}]
# Matches any of the characters '()*+,-./0123456789:;<=>?
# even on an EBCDIC platform.
[\N{U+27}-\N{U+3F}] # Same. (U+27 is "'", and U+3F is "?")
As the final two examples above show, you can achieve portablity to
non-ASCII platforms by using the "\N{...}" form for the range
endpoints. These indicate that the specified range is to be
interpreted using Unicode values, so "[\N{U+27}-\N{U+3F}]" means to
match "\N{U+27}", "\N{U+28}", "\N{U+29}", ..., "\N{U+3D}", "\N{U+3E}",
and "\N{U+3F}", whatever the native code point versions for those are.
These are called "Unicode" ranges. If either end is of the "\N{...}"
form, the range is considered Unicode. A "regexp" warning is raised
under "usere'strict'" if the other endpoint is specified non-
portably:
[\N{U+00}-\x09] # Warning under re 'strict'; \x09 is non-portable
[\N{U+00}-\t] # No warning;
Both of the above match the characters "\N{U+00}" "\N{U+01}", ...
"\N{U+08}", "\N{U+09}", but the "\x09" looks like it could be a mistake
so the warning is raised (under "re 'strict'") for it.
Perl also guarantees that the ranges "A-Z", "a-z", "0-9", and any
subranges of these match what an English-only speaker would expect them
to match on any platform. That is, "[A-Z]" matches the 26 ASCII
uppercase letters; "[a-z]" matches the 26 lowercase letters; and
"[0-9]" matches the 10 digits. Subranges, like "[h-k]", match
correspondingly, in this case just the four letters "h", "i", "j", and
"k". This is the natural behavior on ASCII platforms where the code
points (ordinal values) for "h" through "k" are consecutive integers
(0x68 through 0x6B). But special handling to achieve this may be
needed on platforms with a non-ASCII native character set. For
example, on EBCDIC platforms, the code point for "h" is 0x88, "i" is
0x89, "j" is 0x91, and "k" is 0x92. Perl specially treats "[h-k]" to
exclude the seven code points in the gap: 0x8A through 0x90. This
special handling is only invoked when the range is a subrange of one of
the ASCII uppercase, lowercase, and digit ranges, AND each end of the
range is expressed either as a literal, like "A", or as a named
character ("\N{...}", including the "\N{U+..." form).
EBCDIC Examples:
[i-j] # Matches either "i" or "j"
[i-\N{LATIN SMALL LETTER J}] # Same
[i-\N{U+6A}] # Same
[\N{U+69}-\N{U+6A}] # Same
[\x{89}-\x{91}] # Matches 0x89 ("i"), 0x8A .. 0x90, 0x91 ("j")
[i-\x{91}] # Same
[\x{89}-j] # Same
[i-J] # Matches, 0x89 ("i") .. 0xC1 ("J"); special
# handling doesn't apply because range is mixed
# case
Negation
It is also possible to instead list the characters you do not want to
match. You can do so by using a caret ("^") as the first character in
the character class. For instance, "[^a-z]" matches any character that
is not a lowercase ASCII letter, which therefore includes more than a
million Unicode code points. The class is said to be "negated" or
"inverted".
This syntax make the caret a special character inside a bracketed
character class, but only if it is the first character of the class. So
if you want the caret as one of the characters to match, either escape
the caret or else don't list it first.
In inverted bracketed character classes, Perl ignores the Unicode rules
that normally say that named sequence, and certain characters should
match a sequence of multiple characters use under caseless "/i"
matching. Following those rules could lead to highly confusing
situations:
"ss" =~ /^[^\xDF]+$/ui; # Matches!
This should match any sequences of characters that aren't "\xDF" nor
what "\xDF" matches under "/i". "s" isn't "\xDF", but Unicode says
that "ss" is what "\xDF" matches under "/i". So which one "wins"? Do
you fail the match because the string has "ss" or accept it because it
has an "s" followed by another "s"? Perl has chosen the latter. (See
note in "Bracketed Character Classes" above.)
Examples:
"e" =~ /[^aeiou]/ # No match, the 'e' is listed.
"x" =~ /[^aeiou]/ # Match, as 'x' isn't a lowercase vowel.
"^" =~ /[^^]/ # No match, matches anything that isn't a caret.
"^" =~ /[x^]/ # Match, caret is not special here.
Backslash Sequences
You can put any backslash sequence character class (with the exception
of "\N" and "\R") inside a bracketed character class, and it will act
just as if you had put all characters matched by the backslash sequence
inside the character class. For instance, "[a-f\d]" matches any decimal
digit, or any of the lowercase letters between 'a' and 'f' inclusive.
"\N" within a bracketed character class must be of the forms "\N{name}"
or "\N{U+hex char}", and NOT be the form that matches non-newlines, for
the same reason that a dot "." inside a bracketed character class loses
its special meaning: it matches nearly anything, which generally isn't
what you want to happen.
Examples:
/[\p{Thai}\d]/ # Matches a character that is either a Thai
# character, or a digit.
/[^\p{Arabic}()]/ # Matches a character that is neither an Arabic
# character, nor a parenthesis.
Backslash sequence character classes cannot form one of the endpoints
of a range. Thus, you can't say:
/[\p{Thai}-\d]/ # Wrong!
POSIX Character Classes
POSIX character classes have the form "[:class:]", where class is the
name, and the "[:" and ":]" delimiters. POSIX character classes only
appear inside bracketed character classes, and are a convenient and
descriptive way of listing a group of characters.
Be careful about the syntax,
# Correct:
$string =~ /[[:alpha:]]/
# Incorrect (will warn):
$string =~ /[:alpha:]/
The latter pattern would be a character class consisting of a colon,
and the letters "a", "l", "p" and "h".
POSIX character classes can be part of a larger bracketed character
class. For example,
[01[:alpha:]%]
is valid and matches '0', '1', any alphabetic character, and the
percent sign.
Perl recognizes the following POSIX character classes:
alpha Any alphabetical character ("[A-Za-z]").
alnum Any alphanumeric character ("[A-Za-z0-9]").
ascii Any character in the ASCII character set.
blank A GNU extension, equal to a space or a horizontal tab ("\t").
cntrl Any control character. See Note [2] below.
digit Any decimal digit ("[0-9]"), equivalent to "\d".
graph Any printable character, excluding a space. See Note [3] below.
lower Any lowercase character ("[a-z]").
print Any printable character, including a space. See Note [4] below.
punct Any graphical character excluding "word" characters. Note [5].
space Any whitespace character. "\s" including the vertical tab
("\cK").
upper Any uppercase character ("[A-Z]").
word A Perl extension ("[A-Za-z0-9_]"), equivalent to "\w".
xdigit Any hexadecimal digit ("[0-9a-fA-F]").
Like the Unicode properties, most of the POSIX properties match the
same regardless of whether case-insensitive ("/i") matching is in
effect or not. The two exceptions are "[:upper:]" and "[:lower:]".
Under "/i", they each match the union of "[:upper:]" and "[:lower:]".
Most POSIX character classes have two Unicode-style "\p" property
counterparts. (They are not official Unicode properties, but Perl
extensions derived from official Unicode properties.) The table below
shows the relation between POSIX character classes and these
counterparts.
One counterpart, in the column labelled "ASCII-range Unicode" in the
table, matches only characters in the ASCII character set.
The other counterpart, in the column labelled "Full-range Unicode",
matches any appropriate characters in the full Unicode character set.
For example, "\p{Alpha}" matches not just the ASCII alphabetic
characters, but any character in the entire Unicode character set
considered alphabetic. An entry in the column labelled "backslash
sequence" is a (short) equivalent.
[[:...:]] ASCII-range Full-range backslash Note
Unicode Unicode sequence
-----------------------------------------------------
alpha \p{PosixAlpha} \p{XPosixAlpha}
alnum \p{PosixAlnum} \p{XPosixAlnum}
ascii \p{ASCII}
blank \p{PosixBlank} \p{XPosixBlank} \h [1]
or \p{HorizSpace} [1]
cntrl \p{PosixCntrl} \p{XPosixCntrl} [2]
digit \p{PosixDigit} \p{XPosixDigit} \d
graph \p{PosixGraph} \p{XPosixGraph} [3]
lower \p{PosixLower} \p{XPosixLower}
print \p{PosixPrint} \p{XPosixPrint} [4]
punct \p{PosixPunct} \p{XPosixPunct} [5]
\p{PerlSpace} \p{XPerlSpace} \s [6]
space \p{PosixSpace} \p{XPosixSpace} [6]
upper \p{PosixUpper} \p{XPosixUpper}
word \p{PosixWord} \p{XPosixWord} \w
xdigit \p{PosixXDigit} \p{XPosixXDigit}
[1] "\p{Blank}" and "\p{HorizSpace}" are synonyms.
[2] Control characters don't produce output as such, but instead
usually control the terminal somehow: for example, newline and
backspace are control characters. On ASCII platforms, in the ASCII
range, characters whose code points are between 0 and 31 inclusive,
plus 127 ("DEL") are control characters; on EBCDIC platforms, their
counterparts are control characters.
[3] Any character that is graphical, that is, visible. This class
consists of all alphanumeric characters and all punctuation
characters.
[4] All printable characters, which is the set of all graphical
characters plus those whitespace characters which are not also
controls.
[5] "\p{PosixPunct}" and "[[:punct:]]" in the ASCII range match all
non-controls, non-alphanumeric, non-space characters:
"[-!"#$%&'()*+,./:;<=>?@[\\\]^_`{|}~]" (although if a locale is in
effect, it could alter the behavior of "[[:punct:]]").
The similarly named property, "\p{Punct}", matches a somewhat
different set in the ASCII range, namely
"[-!"#%&'()*,./:;?@[\\\]_{}]". That is, it is missing the nine
characters "[$+<=>^`|~]". This is because Unicode splits what
POSIX considers to be punctuation into two categories, Punctuation
and Symbols.
"\p{XPosixPunct}" and (under Unicode rules) "[[:punct:]]", match
what "\p{PosixPunct}" matches in the ASCII range, plus what
"\p{Punct}" matches. This is different than strictly matching
according to "\p{Punct}". Another way to say it is that if Unicode
rules are in effect, "[[:punct:]]" matches all characters that
Unicode considers punctuation, plus all ASCII-range characters that
Unicode considers symbols.
[6] "\p{XPerlSpace}" and "\p{Space}" match identically starting with
Perl v5.18. In earlier versions, these differ only in that in non-
locale matching, "\p{XPerlSpace}" did not match the vertical tab,
"\cK". Same for the two ASCII-only range forms.
There are various other synonyms that can be used besides the names
listed in the table. For example, "\p{PosixAlpha}" can be written as
"\p{Alpha}". All are listed in "Properties accessible through \p{} and
\P{}" in perluniprops.
Both the "\p" counterparts always assume Unicode rules are in effect.
On ASCII platforms, this means they assume that the code points from
128 to 255 are Latin-1, and that means that using them under locale
rules is unwise unless the locale is guaranteed to be Latin-1 or UTF-8.
In contrast, the POSIX character classes are useful under locale rules.
They are affected by the actual rules in effect, as follows:
If the "/a" modifier, is in effect ...
Each of the POSIX classes matches exactly the same as their ASCII-
range counterparts.
otherwise ...
For code points above 255 ...
The POSIX class matches the same as its Full-range counterpart.
For code points below 256 ...
if locale rules are in effect ...
The POSIX class matches according to the locale, except:
"word"
also includes the platform's native underscore
character, no matter what the locale is.
"ascii"
on platforms that don't have the POSIX "ascii"
extension, this matches just the platform's native
ASCII-range characters.
"blank"
on platforms that don't have the POSIX "blank"
extension, this matches just the platform's native tab
and space characters.
if Unicode rules are in effect ...
The POSIX class matches the same as the Full-range
counterpart.
otherwise ...
The POSIX class matches the same as the ASCII range
counterpart.
Which rules apply are determined as described in "Which character set
modifier is in effect?" in perlre.
It is proposed to change this behavior in a future release of Perl so
that whether or not Unicode rules are in effect would not change the
behavior: Outside of locale, the POSIX classes would behave like their
ASCII-range counterparts. If you wish to comment on this proposal,
send email to "perl5-porters@perl.org".
Negation of POSIX character classes
A Perl extension to the POSIX character class is the ability to negate
it. This is done by prefixing the class name with a caret ("^"). Some
examples:
POSIX ASCII-range Full-range backslash
Unicode Unicode sequence
-----------------------------------------------------
[[:^digit:]] \P{PosixDigit} \P{XPosixDigit} \D
[[:^space:]] \P{PosixSpace} \P{XPosixSpace}
\P{PerlSpace} \P{XPerlSpace} \S
[[:^word:]] \P{PerlWord} \P{XPosixWord} \W
The backslash sequence can mean either ASCII- or Full-range Unicode,
depending on various factors as described in "Which character set
modifier is in effect?" in perlre.
[= =] and [. .]
Perl recognizes the POSIX character classes "[=class=]" and
"[.class.]", but does not (yet?) support them. Any attempt to use
either construct raises an exception.
Examples
/[[:digit:]]/ # Matches a character that is a digit.
/[01[:lower:]]/ # Matches a character that is either a
# lowercase letter, or '0' or '1'.
/[[:digit:][:^xdigit:]]/ # Matches a character that can be anything
# except the letters 'a' to 'f' and 'A' to
# 'F'. This is because the main character
# class is composed of two POSIX character
# classes that are ORed together, one that
# matches any digit, and the other that
# matches anything that isn't a hex digit.
# The OR adds the digits, leaving only the
# letters 'a' to 'f' and 'A' to 'F' excluded.
Extended Bracketed Character Classes
This is a fancy bracketed character class that can be used for more
readable and less error-prone classes, and to perform set operations,
such as intersection. An example is
/(?[ \p{Thai} & \p{Digit} ])/
This will match all the digit characters that are in the Thai script.
This is an experimental feature available starting in 5.18, and is
subject to change as we gain field experience with it. Any attempt to
use it will raise a warning, unless disabled via
no warnings "experimental::regex_sets";
Comments on this feature are welcome; send email to
"perl5-porters@perl.org".
The rules used by "use re 'strict" apply to this construct.
We can extend the example above:
/(?[ ( \p{Thai} + \p{Lao} ) & \p{Digit} ])/
This matches digits that are in either the Thai or Laotian scripts.
Notice the white space in these examples. This construct always has
the "/x" modifier turned on within it.
The available binary operators are:
& intersection
+ union
| another name for '+', hence means union
- subtraction (the result matches the set consisting of those
code points matched by the first operand, excluding any that
are also matched by the second operand)
^ symmetric difference (the union minus the intersection). This
is like an exclusive or, in that the result is the set of code
points that are matched by either, but not both, of the
operands.
There is one unary operator:
! complement
All the binary operators left associate; "&" is higher precedence than
the others, which all have equal precedence. The unary operator right
associates, and has highest precedence. Thus this follows the normal
Perl precedence rules for logical operators. Use parentheses to
override the default precedence and associativity.
The main restriction is that everything is a metacharacter. Thus, you
cannot refer to single characters by doing something like this:
/(?[ a + b ])/ # Syntax error!
The easiest way to specify an individual typable character is to
enclose it in brackets:
/(?[ [a] + [b] ])/
(This is the same thing as "[ab]".) You could also have said the
equivalent:
/(?[[ a b ]])/
(You can, of course, specify single characters by using, "\x{...}",
"\N{...}", etc.)
This last example shows the use of this construct to specify an
ordinary bracketed character class without additional set operations.
Note the white space within it; a limited version of "/x" is turned on
even within bracketed character classes, with only the SPACE and TAB
("\t") characters allowed, and no comments. Hence,
(?[ [#] ])
matches the literal character "#". To specify a literal white space
character, you can escape it with a backslash, like:
/(?[ [ a e i o u \ ] ])/
This matches the English vowels plus the SPACE character. All the
other escapes accepted by normal bracketed character classes are
accepted here as well; but unrecognized escapes that generate warnings
in normal classes are fatal errors here.
All warnings from these class elements are fatal, as well as some
practices that don't currently warn. For example you cannot say
/(?[ [ \xF ] ])/ # Syntax error!
You have to have two hex digits after a braceless "\x" (use a leading
zero to make two). These restrictions are to lower the incidence of
typos causing the class to not match what you thought it would.
If a regular bracketed character class contains a "\p{}" or "\P{}" and
is matched against a non-Unicode code point, a warning may be raised,
as the result is not Unicode-defined. No such warning will come when
using this extended form.
The final difference between regular bracketed character classes and
these, is that it is not possible to get these to match a multi-
character fold. Thus,
/(?[ [\xDF] ])/iu
does not match the string "ss".
You don't have to enclose POSIX class names inside double brackets,
hence both of the following work:
/(?[ [:word:] - [:lower:] ])/
/(?[ [[:word:]] - [[:lower:]] ])/
Any contained POSIX character classes, including things like "\w" and
"\D" respect the "/a" (and "/aa") modifiers.
"(?[ ])" is a regex-compile-time construct. Any attempt to use
something which isn't knowable at the time the containing regular
expression is compiled is a fatal error. In practice, this means just
three limitations:
1. When compiled within the scope of "use locale" (or the "/l" regex
modifier), this construct assumes that the execution-time locale
will be a UTF-8 one, and the generated pattern always uses Unicode
rules. What gets matched or not thus isn't dependent on the actual
runtime locale, so tainting is not enabled. But a "locale"
category warning is raised if the runtime locale turns out to not
be UTF-8.
2. Any user-defined property used must be already defined by the time
the regular expression is compiled (but note that this construct
can be used instead of such properties).
3. A regular expression that otherwise would compile using "/d" rules,
and which uses this construct will instead use "/u". Thus this
construct tells Perl that you don't want "/d" rules for the entire
regular expression containing it.
Note that skipping white space applies only to the interior of this
construct. There must not be any space between any of the characters
that form the initial "(?[". Nor may there be space between the
closing "])" characters.
Just as in all regular expressions, the pattern can be built up by
including variables that are interpolated at regex compilation time.
Care must be taken to ensure that you are getting what you expect. For
example:
my $thai_or_lao = '\p{Thai} + \p{Lao}';
...
qr/(?[ \p{Digit} & $thai_or_lao ])/;
compiles to
qr/(?[ \p{Digit} & \p{Thai} + \p{Lao} ])/;
But this does not have the effect that someone reading the code would
likely expect, as the intersection applies just to "\p{Thai}",
excluding the Laotian. Pitfalls like this can be avoided by
parenthesizing the component pieces:
my $thai_or_lao = '( \p{Thai} + \p{Lao} )';
But any modifiers will still apply to all the components:
my $lower = '\p{Lower} + \p{Digit}';
qr/(?[ \p{Greek} & $lower ])/i;
matches upper case things. You can avoid surprises by making the
components into instances of this construct by compiling them:
my $thai_or_lao = qr/(?[ \p{Thai} + \p{Lao} ])/;
my $lower = qr/(?[ \p{Lower} + \p{Digit} ])/;
When these are embedded in another pattern, what they match does not
change, regardless of parenthesization or what modifiers are in effect
in that outer pattern.
Due to the way that Perl parses things, your parentheses and brackets
may need to be balanced, even including comments. If you run into any
examples, please send them to "perlbug@perl.org", so that we can have a
concrete example for this man page.
We may change it so that things that remain legal uses in normal
bracketed character classes might become illegal within this
experimental construct. One proposal, for example, is to forbid
adjacent uses of the same character, as in "(?[ [aa] ])". The
motivation for such a change is that this usage is likely a typo, as
the second "a" adds nothing.
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