perlunicode - Unicode support in Perl


   If you haven't already, before reading this document, you should become
   familiar with both perlunitut and perluniintro.

   Unicode aims to UNI-fy the en-CODE-ings of all the world's character
   sets into a single Standard.   For quite a few of the various coding
   standards that existed when Unicode was first created, converting from
   each to Unicode essentially meant adding a constant to each code point
   in the original standard, and converting back meant just subtracting
   that same constant.  For ASCII and ISO-8859-1, the constant is 0.  For
   ISO-8859-5, (Cyrillic) the constant is 864; for Hebrew (ISO-8859-8),
   it's 1488; Thai (ISO-8859-11), 3424; and so forth.  This made it easy
   to do the conversions, and facilitated the adoption of Unicode.

   And it worked; nowadays, those legacy standards are rarely used.  Most
   everyone uses Unicode.

   Unicode is a comprehensive standard.  It specifies many things outside
   the scope of Perl, such as how to display sequences of characters.  For
   a full discussion of all aspects of Unicode, see

   Important Caveats
   Even though some of this section may not be understandable to you on
   first reading, we think it's important enough to highlight some of the
   gotchas before delving further, so here goes:

   Unicode support is an extensive requirement. While Perl does not
   implement the Unicode standard or the accompanying technical reports
   from cover to cover, Perl does support many Unicode features.

   Also, the use of Unicode may present security issues that aren't
   obvious.  Read Unicode Security Considerations

   Safest if you "use feature 'unicode_strings'"
       In order to preserve backward compatibility, Perl does not turn on
       full internal Unicode support unless the pragma
       "usefeature'unicode_strings'" is specified.  (This is
       automatically selected if you "use5.012" or higher.)  Failure to
       do this can trigger unexpected surprises.  See "The "Unicode Bug""

       This pragma doesn't affect I/O.  Nor does it change the internal
       representation of strings, only their interpretation.  There are
       still several places where Unicode isn't fully supported, such as
       in filenames.

   Input and Output Layers
       Use the ":encoding(...)" layer  to read from and write to
       filehandles using the specified encoding.  (See open.)

   You should convert your non-ASCII, non-UTF-8 Perl scripts to be UTF-8.
       See encoding.

   "use utf8" still needed to enable UTF-8 in scripts
       If your Perl script is itself encoded in UTF-8, the "useutf8"
       pragma must be explicitly included to enable recognition of that
       (in string or regular expression literals, or in identifier names).
       This is the only time when an explicit "useutf8" is needed.  (See

   "BOM"-marked scripts and UTF-16 scripts autodetected
       However, if a Perl script begins with the Unicode "BOM" (UTF-16LE,
       UTF16-BE, or UTF-8), or if the script looks like non-"BOM"-marked
       UTF-16 of either endianness, Perl will correctly read in the script
       as the appropriate Unicode encoding.  ("BOM"-less UTF-8 cannot be
       effectively recognized or differentiated from ISO 8859-1 or other
       eight-bit encodings.)

   Byte and Character Semantics
   Before Unicode, most encodings used 8 bits (a single byte) to encode
   each character.  Thus a character was a byte, and a byte was a
   character, and there could be only 256 or fewer possible characters.
   "Byte Semantics" in the title of this section refers to this behavior.
   There was no need to distinguish between "Byte" and "Character".

   Then along comes Unicode which has room for over a million characters
   (and Perl allows for even more).  This means that a character may
   require more than a single byte to represent it, and so the two terms
   are no longer equivalent.  What matter are the characters as whole
   entities, and not usually the bytes that comprise them.  That's what
   the term "Character Semantics" in the title of this section refers to.

   Perl had to change internally to decouple "bytes" from "characters".
   It is important that you too change your ideas, if you haven't already,
   so that "byte" and "character" no longer mean the same thing in your

   The basic building block of Perl strings has always been a "character".
   The changes basically come down to that the implementation no longer
   thinks that a character is always just a single byte.

   There are various things to note:

   *   String handling functions, for the most part, continue to operate
       in terms of characters.  "length()", for example, returns the
       number of characters in a string, just as before.  But that number
       no longer is necessarily the same as the number of bytes in the
       string (there may be more bytes than characters).  The other such
       functions include "chop()", "chomp()", "substr()", "pos()",
       "index()", "rindex()", "sort()", "sprintf()", and "write()".

       The exceptions are:

       *   the bit-oriented "vec"


       *   the byte-oriented "pack"/"unpack" "C" format

           However, the "W" specifier does operate on whole characters, as
           does the "U" specifier.

       *   some operators that interact with the platform's operating

           Operators dealing with filenames are examples.

       *   when the functions are called from within the scope of the
           "usebytes" pragma

           Likely, you should use this only for debugging anyway.

   *   Strings--including hash keys--and regular expression patterns may
       contain characters that have ordinal values larger than 255.

       If you use a Unicode editor to edit your program, Unicode
       characters may occur directly within the literal strings in UTF-8
       encoding, or UTF-16.  (The former requires a "BOM" or "use utf8",
       the latter requires a "BOM".)

       "Creating Unicode" in perluniintro gives other ways to place non-
       ASCII characters in your strings.

   *   The "chr()" and "ord()" functions work on whole characters.

   *   Regular expressions match whole characters.  For example, "."
       matches a whole character instead of only a single byte.

   *   The "tr///" operator translates whole characters.  (Note that the
       "tr///CU" functionality has been removed.  For similar
       functionality to that, see "pack('U0', ...)" and "pack('C0',

   *   "scalar reverse()" reverses by character rather than by byte.

   *   The bit string operators, "& | ^ ~" and (starting in v5.22) "&. |.
       ^.  ~." can operate on characters that don't fit into a byte.
       However, the current behavior is likely to change.  You should not
       use these operators on strings that are encoded in UTF-8.  If
       you're not sure about the encoding of a string, downgrade it before
       using any of these operators; you can use "utf8::utf8_downgrade()".

   The bottom line is that Perl has always practiced "Character
   Semantics", but with the advent of Unicode, that is now different than
   "Byte Semantics".

   ASCII Rules versus Unicode Rules
   Before Unicode, when a character was a byte was a character, Perl knew
   only about the 128 characters defined by ASCII, code points 0 through
   127 (except for under "uselocale").  That left the code points 128 to
   255 as unassigned, and available for whatever use a program might want.
   The only semantics they have is their ordinal numbers, and that they
   are members of none of the non-negative character classes.  None are
   considered to match "\w" for example, but all match "\W".

   Unicode, of course, assigns each of those code points a particular
   meaning (along with ones above 255).  To preserve backward
   compatibility, Perl only uses the Unicode meanings when there is some
   indication that Unicode is what is intended; otherwise the non-ASCII
   code points remain treated as if they are unassigned.

   Here are the ways that Perl knows that a string should be treated as

   *   Within the scope of "useutf8"

       If the whole program is Unicode (signified by using 8-bit Unicode
       Transformation Format), then all strings within it must be Unicode.

   *   Within the scope of "usefeature'unicode_strings'"

       This pragma was created so you can explicitly tell Perl that
       operations executed within its scope are to use Unicode rules.
       More operations are affected with newer perls.  See "The "Unicode

   *   Within the scope of "use5.012" or higher

       This implicitly turns on "usefeature'unicode_strings'".

   *   Within the scope of "uselocale'not_characters'", or "uselocale"
       and the current locale is a UTF-8 locale.

       The former is defined to imply Unicode handling; and the latter
       indicates a Unicode locale, hence a Unicode interpretation of all
       strings within it.

   *   When the string contains a Unicode-only code point

       Perl has never accepted code points above 255 without them being
       Unicode, so their use implies Unicode for the whole string.

   *   When the string contains a Unicode named code point "\N{...}"

       The "\N{...}" construct explicitly refers to a Unicode code point,
       even if it is one that is also in ASCII.  Therefore the string
       containing it must be Unicode.

   *   When the string has come from an external source marked as Unicode

       The "-C" command line option can specify that certain inputs to the
       program are Unicode, and the values of this can be read by your
       Perl code, see "${^UNICODE}" in perlvar.

   *   When the string has been upgraded to UTF-8

       The function "utf8::utf8_upgrade()" can be explicitly used to
       permanently (unless a subsequent "utf8::utf8_downgrade()" is
       called) cause a string to be treated as Unicode.

   *   There are additional methods for regular expression patterns

       A pattern that is compiled with the "/u" or "/a" modifiers is
       treated as Unicode (though there are some restrictions with "/a").
       Under the "/d" and "/l" modifiers, there are several other
       indications for Unicode; see "Character set modifiers" in perlre.

   Note that all of the above are overridden within the scope of "use
   bytes"; but you should be using this pragma only for debugging.

   Note also that some interactions with the platform's operating system
   never use Unicode rules.

   When Unicode rules are in effect:

   *   Case translation operators use the Unicode case translation tables.

       Note that "uc()", or "\U" in interpolated strings, translates to
       uppercase, while "ucfirst", or "\u" in interpolated strings,
       translates to titlecase in languages that make the distinction
       (which is equivalent to uppercase in languages without the

       There is a CPAN module, "Unicode::Casing", which allows you to
       define your own mappings to be used in "lc()", "lcfirst()", "uc()",
       "ucfirst()", and "fc" (or their double-quoted string inlined
       versions such as "\U").  (Prior to Perl 5.16, this functionality
       was partially provided in the Perl core, but suffered from a number
       of insurmountable drawbacks, so the CPAN module was written

   *   Character classes in regular expressions match based on the
       character properties specified in the Unicode properties database.

       "\w" can be used to match a Japanese ideograph, for instance; and
       "[[:digit:]]" a Bengali number.

   *   Named Unicode properties, scripts, and block ranges may be used
       (like bracketed character classes) by using the "\p{}" "matches
       property" construct and the "\P{}" negation, "doesn't match

       See "Unicode Character Properties" for more details.

       You can define your own character properties and use them in the
       regular expression with the "\p{}" or "\P{}" construct.  See "User-
       Defined Character Properties" for more details.

   Extended Grapheme Clusters (Logical characters)
   Consider a character, say "H".  It could appear with various marks
   around it, such as an acute accent, or a circumflex, or various hooks,
   circles, arrows, etc., above, below, to one side or the other, etc.
   There are many possibilities among the world's languages.  The number
   of combinations is astronomical, and if there were a character for each
   combination, it would soon exhaust Unicode's more than a million
   possible characters.  So Unicode took a different approach: there is a
   character for the base "H", and a character for each of the possible
   marks, and these can be variously combined to get a final logical
   character.  So a logical character--what appears to be a single
   character--can be a sequence of more than one individual characters.
   The Unicode standard calls these "extended grapheme clusters" (which is
   an improved version of the no-longer much used "grapheme cluster");
   Perl furnishes the "\X" regular expression construct to match such
   sequences in their entirety.

   But Unicode's intent is to unify the existing character set standards
   and practices, and several pre-existing standards have single
   characters that mean the same thing as some of these combinations, like
   ISO-8859-1, which has quite a few of them. For example, "LATIN CAPITAL
   LETTER E WITH ACUTE" was already in this standard when Unicode came
   along.  Unicode therefore added it to its repertoire as that single
   character.  But this character is considered by Unicode to be
   equivalent to the sequence consisting of the character "LATIN CAPITAL
   LETTER E" followed by the character "COMBINING ACUTE ACCENT".

   "LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed"
   character, and its equivalence with the "E" and the "COMBINING ACCENT"
   sequence is called canonical equivalence.  All pre-composed characters
   are said to have a decomposition (into the equivalent sequence), and
   the decomposition type is also called canonical.  A string may be
   comprised as much as possible of precomposed characters, or it may be
   comprised of entirely decomposed characters.  Unicode calls these
   respectively, "Normalization Form Composed" (NFC) and "Normalization
   Form Decomposed".  The "Unicode::Normalize" module contains functions
   that convert between the two.  A string may also have both composed
   characters and decomposed characters; this module can be used to make
   it all one or the other.

   You may be presented with strings in any of these equivalent forms.
   There is currently nothing in Perl 5 that ignores the differences.  So
   you'll have to specially hanlde it.  The usual advice is to convert
   your inputs to "NFD" before processing further.

   For more detailed information, see <>.

   Unicode Character Properties
   (The only time that Perl considers a sequence of individual code points
   as a single logical character is in the "\X" construct, already
   mentioned above.   Therefore "character" in this discussion means a
   single Unicode code point.)

   Very nearly all Unicode character properties are accessible through
   regular expressions by using the "\p{}" "matches property" construct
   and the "\P{}" "doesn't match property" for its negation.

   For instance, "\p{Uppercase}" matches any single character with the
   Unicode "Uppercase" property, while "\p{L}" matches any character with
   a "General_Category" of "L" (letter) property (see "General_Category"
   below).  Brackets are not required for single letter property names, so
   "\p{L}" is equivalent to "\pL".

   More formally, "\p{Uppercase}" matches any single character whose
   Unicode "Uppercase" property value is "True", and "\P{Uppercase}"
   matches any character whose "Uppercase" property value is "False", and
   they could have been written as "\p{Uppercase=True}" and
   "\p{Uppercase=False}", respectively.

   This formality is needed when properties are not binary; that is, if
   they can take on more values than just "True" and "False".  For
   example, the "Bidi_Class" property (see "Bidirectional Character Types"
   below), can take on several different values, such as "Left", "Right",
   "Whitespace", and others.  To match these, one needs to specify both
   the property name ("Bidi_Class"), AND the value being matched against
   ("Left", "Right", etc.).  This is done, as in the examples above, by
   having the two components separated by an equal sign (or
   interchangeably, a colon), like "\p{Bidi_Class: Left}".

   All Unicode-defined character properties may be written in these
   compound forms of "\p{property=value}" or "\p{property:value}", but
   Perl provides some additional properties that are written only in the
   single form, as well as single-form short-cuts for all binary
   properties and certain others described below, in which you may omit
   the property name and the equals or colon separator.

   Most Unicode character properties have at least two synonyms (or
   aliases if you prefer): a short one that is easier to type and a longer
   one that is more descriptive and hence easier to understand.  Thus the
   "L" and "Letter" properties above are equivalent and can be used
   interchangeably.  Likewise, "Upper" is a synonym for "Uppercase", and
   we could have written "\p{Uppercase}" equivalently as "\p{Upper}".
   Also, there are typically various synonyms for the values the property
   can be.   For binary properties, "True" has 3 synonyms: "T", "Yes", and
   "Y"; and "False" has correspondingly "F", "No", and "N".  But be
   careful.  A short form of a value for one property may not mean the
   same thing as the same short form for another.  Thus, for the
   "General_Category" property, "L" means "Letter", but for the
   "Bidi_Class" property, "L" means "Left".  A complete list of properties
   and synonyms is in perluniprops.

   Upper/lower case differences in property names and values are
   irrelevant; thus "\p{Upper}" means the same thing as "\p{upper}" or
   even "\p{UpPeR}".  Similarly, you can add or subtract underscores
   anywhere in the middle of a word, so that these are also equivalent to
   "\p{U_p_p_e_r}".  And white space is irrelevant adjacent to non-word
   characters, such as the braces and the equals or colon separators, so
   "\p{   Upper  }" and "\p{ Upper_case : Y }" are equivalent to these as
   well.  In fact, white space and even hyphens can usually be added or
   deleted anywhere.  So even "\p{ Up-per case = Yes}" is equivalent.  All
   this is called "loose-matching" by Unicode.  The few places where
   stricter matching is used is in the middle of numbers, and in the Perl
   extension properties that begin or end with an underscore.  Stricter
   matching cares about white space (except adjacent to non-word
   characters), hyphens, and non-interior underscores.

   You can also use negation in both "\p{}" and "\P{}" by introducing a
   caret ("^") between the first brace and the property name: "\p{^Tamil}"
   is equal to "\P{Tamil}".

   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.  And the second set is
   "Uppercase", "Lowercase", and "Titlecase", all of which match "Cased"
   under "/i" matching.  This set also includes its subsets "PosixUpper"
   and "PosixLower" both of which under "/i" match "PosixAlpha".  (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 letters, so they aren't "Cased_Letter"'s.)

   See "Beyond Unicode code points" for special considerations when
   matching Unicode properties against non-Unicode code points.


   Every Unicode character is assigned a general category, which is the
   "most usual categorization of a character" (from

   The compound way of writing these is like "\p{General_Category=Number}"
   (short: "\p{gc:n}").  But Perl furnishes shortcuts in which everything
   up through the equal or colon separator is omitted.  So you can instead
   just write "\pN".

   Here are the short and long forms of the values the "General Category"
   property can have:

       Short       Long

       L           Letter
       LC, L&      Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
       Lu          Uppercase_Letter
       Ll          Lowercase_Letter
       Lt          Titlecase_Letter
       Lm          Modifier_Letter
       Lo          Other_Letter

       M           Mark
       Mn          Nonspacing_Mark
       Mc          Spacing_Mark
       Me          Enclosing_Mark

       N           Number
       Nd          Decimal_Number (also Digit)
       Nl          Letter_Number
       No          Other_Number

       P           Punctuation (also Punct)
       Pc          Connector_Punctuation
       Pd          Dash_Punctuation
       Ps          Open_Punctuation
       Pe          Close_Punctuation
       Pi          Initial_Punctuation
                   (may behave like Ps or Pe depending on usage)
       Pf          Final_Punctuation
                   (may behave like Ps or Pe depending on usage)
       Po          Other_Punctuation

       S           Symbol
       Sm          Math_Symbol
       Sc          Currency_Symbol
       Sk          Modifier_Symbol
       So          Other_Symbol

       Z           Separator
       Zs          Space_Separator
       Zl          Line_Separator
       Zp          Paragraph_Separator

       C           Other
       Cc          Control (also Cntrl)
       Cf          Format
       Cs          Surrogate
       Co          Private_Use
       Cn          Unassigned

   Single-letter properties match all characters in any of the two-letter
   sub-properties starting with the same letter.  "LC" and "L&" are
   special: both are aliases for the set consisting of everything matched
   by "Ll", "Lu", and "Lt".

   Bidirectional Character Types

   Because scripts differ in their directionality (Hebrew and Arabic are
   written right to left, for example) Unicode supplies a "Bidi_Class"
   property.  Some of the values this property can have are:

       Value       Meaning

       L           Left-to-Right
       LRE         Left-to-Right Embedding
       LRO         Left-to-Right Override
       R           Right-to-Left
       AL          Arabic Letter
       RLE         Right-to-Left Embedding
       RLO         Right-to-Left Override
       PDF         Pop Directional Format
       EN          European Number
       ES          European Separator
       ET          European Terminator
       AN          Arabic Number
       CS          Common Separator
       NSM         Non-Spacing Mark
       BN          Boundary Neutral
       B           Paragraph Separator
       S           Segment Separator
       WS          Whitespace
       ON          Other Neutrals

   This property is always written in the compound form.  For example,
   "\p{Bidi_Class:R}" matches characters that are normally written right
   to left.  Unlike the "General_Category" property, this property can
   have more values added in a future Unicode release.  Those listed above
   comprised the complete set for many Unicode releases, but others were
   added in Unicode 6.3; you can always find what the current ones are in
   perluniprops.  And <> describes how
   to use them.


   The world's languages are written in many different scripts.  This
   sentence (unless you're reading it in translation) is written in Latin,
   while Russian is written in Cyrillic, and Greek is written in, well,
   Greek; Japanese mainly in Hiragana or Katakana.  There are many more.

   The Unicode "Script" and "Script_Extensions" properties give what
   script a given character is in.  Either property can be specified with
   the compound form like "\p{Script=Hebrew}" (short: "\p{sc=hebr}"), or
   "\p{Script_Extensions=Javanese}" (short: "\p{scx=java}").  In addition,
   Perl furnishes shortcuts for all "Script" property names.  You can omit
   everything up through the equals (or colon), and simply write
   "\p{Latin}" or "\P{Cyrillic}".  (This is not true for
   "Script_Extensions", which is required to be written in the compound

   The difference between these two properties involves characters that
   are used in multiple scripts.  For example the digits '0' through '9'
   are used in many parts of the world.  These are placed in a script
   named "Common".  Other characters are used in just a few scripts.  For
   example, the "KATAKANA-HIRAGANA DOUBLE HYPHEN" is used in both Japanese
   scripts, Katakana and Hiragana, but nowhere else.  The "Script"
   property places all characters that are used in multiple scripts in the
   "Common" script, while the "Script_Extensions" property places those
   that are used in only a few scripts into each of those scripts; while
   still using "Common" for those used in many scripts.  Thus both these

    "0" =~ /\p{sc=Common}/     # Matches
    "0" =~ /\p{scx=Common}/    # Matches

   and only the first of these match:

    "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Common}  # Matches
    "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Common} # No match

   And only the last two of these match:

    "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Hiragana}  # No match
    "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{sc=Katakana}  # No match
    "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Hiragana} # Matches
    "\N{KATAKANA-HIRAGANA DOUBLE HYPHEN}" =~ /\p{scx=Katakana} # Matches

   "Script_Extensions" is thus an improved "Script", in which there are
   fewer characters in the "Common" script, and correspondingly more in
   other scripts.  It is new in Unicode version 6.0, and its data are
   likely to change significantly in later releases, as things get sorted
   out.  New code should probably be using "Script_Extensions" and not
   plain "Script".

   (Actually, besides "Common", the "Inherited" script, contains
   characters that are used in multiple scripts.  These are modifier
   characters which inherit the script value of the controlling character.
   Some of these are used in many scripts, and so go into "Inherited" in
   both "Script" and "Script_Extensions".  Others are used in just a few
   scripts, so are in "Inherited" in "Script", but not in

   It is worth stressing that there are several different sets of digits
   in Unicode that are equivalent to 0-9 and are matchable by "\d" in a
   regular expression.  If they are used in a single language only, they
   are in that language's "Script" and "Script_Extension".  If they are
   used in more than one script, they will be in "sc=Common", but only if
   they are used in many scripts should they be in "scx=Common".

   A complete list of scripts and their shortcuts is in perluniprops.

   Use of the "Is" Prefix

   For backward compatibility (with Perl 5.6), all properties writable
   without using the compound form mentioned so far may have "Is" or "Is_"
   prepended to their name, so "\P{Is_Lu}", for example, is equal to
   "\P{Lu}", and "\p{IsScript:Arabic}" is equal to "\p{Arabic}".


   In addition to scripts, Unicode also defines blocks of characters.  The
   difference between scripts and blocks is that the concept of scripts is
   closer to natural languages, while the concept of blocks is more of an
   artificial grouping based on groups of Unicode characters with
   consecutive ordinal values. For example, the "Basic Latin" block is all
   the characters whose ordinals are between 0 and 127, inclusive; in
   other words, the ASCII characters.  The "Latin" script contains some
   letters from this as well as several other blocks, like "Latin-1
   Supplement", "Latin Extended-A", etc., but it does not contain all the
   characters from those blocks. It does not, for example, contain the
   digits 0-9, because those digits are shared across many scripts, and
   hence are in the "Common" script.

   For more about scripts versus blocks, see UAX#24 "Unicode Script
   Property": <>

   The "Script" or "Script_Extensions" properties are likely to be the
   ones you want to use when processing natural language; the "Block"
   property may occasionally be useful in working with the nuts and bolts
   of Unicode.

   Block names are matched in the compound form, like "\p{Block: Arrows}"
   or "\p{Blk=Hebrew}".  Unlike most other properties, only a few block
   names have a Unicode-defined short name.

   Perl also defines single form synonyms for the block property in cases
   where these do not conflict with something else.  But don't use any of
   these, because they are unstable.  Since these are Perl extensions,
   they are subordinate to official Unicode property names; Unicode
   doesn't know nor care about Perl's extensions.  It may happen that a
   name that currently means the Perl extension will later be changed
   without warning to mean a different Unicode property in a future
   version of the perl interpreter that uses a later Unicode release, and
   your code would no longer work.  The extensions are mentioned here for
   completeness:  Take the block name and prefix it with one of: "In" (for
   example "\p{Blk=Arrows}" can currently be written as "\p{In_Arrows}");
   or sometimes "Is" (like "\p{Is_Arrows}"); or sometimes no prefix at all
   ("\p{Arrows}").  As of this writing (Unicode 8.0) there are no
   conflicts with using the "In_" prefix, but there are plenty with the
   other two forms.  For example, "\p{Is_Hebrew}" and "\p{Hebrew}" mean
   "\p{Script=Hebrew}" which is NOT the same thing as "\p{Blk=Hebrew}".
   Our advice used to be to use the "In_" prefix as a single form way of
   specifying a block.  But Unicode 8.0 added properties whose names begin
   with "In", and it's now clear that it's only luck that's so far
   prevented a conflict.  Using "In" is only marginally less typing than
   "Blk:", and the latter's meaning is clearer anyway, and guaranteed to
   never conflict.  So don't take chances.  Use "\p{Blk=foo}" for new
   code.  And be sure that block is what you really really want to do.  In
   most cases scripts are what you want instead.

   A complete list of blocks is in perluniprops.

   Other Properties

   There are many more properties than the very basic ones described here.
   A complete list is in perluniprops.

   Unicode defines all its properties in the compound form, so all single-
   form properties are Perl extensions.  Most of these are just synonyms
   for the Unicode ones, but some are genuine extensions, including
   several that are in the compound form.  And quite a few of these are
   actually recommended by Unicode (in

   This section gives some details on all extensions that aren't just
   synonyms for compound-form Unicode properties (for those properties,
   you'll have to refer to the Unicode Standard

       This matches every possible code point.  It is equivalent to
       "qr/./s".  Unlike all the other non-user-defined "\p{}" property
       matches, no warning is ever generated if this is property is
       matched against a non-Unicode code point (see "Beyond Unicode code
       points" below).

       This matches any "\p{Alphabetic}" or "\p{Decimal_Number}"

       This matches any of the 1_114_112 Unicode code points.  It is a
       synonym for "\p{Unicode}".

       This matches any of the 128 characters in the US-ASCII character
       set, which is a subset of Unicode.

       This matches any assigned code point; that is, any code point whose
       general category is not "Unassigned" (or equivalently, not "Cn").

       This is the same as "\h" and "\p{HorizSpace}":  A character that
       changes the spacing horizontally.

   "\p{Decomposition_Type: Non_Canonical}"    (Short: "\p{Dt=NonCanon}")
       Matches a character that has a non-canonical decomposition.

       The "Extended Grapheme Clusters (Logical characters)" section above
       talked about canonical decompositions.  However, many more
       characters have a different type of decomposition, a "compatible"
       or "non-canonical" decomposition.  The sequences that form these
       decompositions are not considered canonically equivalent to the
       pre-composed character.  An example is the "SUPERSCRIPT ONE".  It
       is somewhat like a regular digit 1, but not exactly; its
       decomposition into the digit 1 is called a "compatible"
       decomposition, specifically a "super" decomposition.  There are
       several such compatibility decompositions (see
       <>), including one called
       "compat", which means some miscellaneous type of decomposition that
       doesn't fit into the other decomposition categories that Unicode
       has chosen.

       Note that most Unicode characters don't have a decomposition, so
       their decomposition type is "None".

       For your convenience, Perl has added the "Non_Canonical"
       decomposition type to mean any of the several compatibility

       Matches any character that is graphic.  Theoretically, this means a
       character that on a printer would cause ink to be used.

       This is the same as "\h" and "\p{Blank}":  a character that changes
       the spacing horizontally.

       This is a synonym for "\p{Present_In=*}"

       This is the same as "\s", restricted to ASCII, namely "[\f\n\r\t]"
       and starting in Perl v5.18, a vertical tab.

       Mnemonic: Perl's (original) space

       This is the same as "\w", restricted to ASCII, namely

       Mnemonic: Perl's (original) word.

       There are several of these, which are equivalents, using the "\p{}"
       notation, for Posix classes and are described in "POSIX Character
       Classes" in perlrecharclass.

   "\p{Present_In: *}"    (Short: "\p{In=*}")
       This property is used when you need to know in what Unicode
       version(s) a character is.

       The "*" above stands for some two digit Unicode version number,
       such as 1.1 or 4.0; or the "*" can also be "Unassigned".  This
       property will match the code points whose final disposition has
       been settled as of the Unicode release given by the version number;
       "\p{Present_In: Unassigned}" will match those code points whose
       meaning has yet to be assigned.

       For example, "U+0041" "LATIN CAPITAL LETTER A" was present in the
       very first Unicode release available, which is 1.1, so this
       property is true for all valid "*" versions.  On the other hand,
       "U+1EFF" was not assigned until version 5.1 when it became "LATIN
       SMALL LETTER Y WITH LOOP", so the only "*" that would match it are
       5.1, 5.2, and later.

       Unicode furnishes the "Age" property from which this is derived.
       The problem with Age is that a strict interpretation of it (which
       Perl takes) has it matching the precise release a code point's
       meaning is introduced in.  Thus "U+0041" would match only 1.1; and
       "U+1EFF" only 5.1.  This is not usually what you want.

       Some non-Perl implementations of the Age property may change its
       meaning to be the same as the Perl "Present_In" property; just be
       aware of that.

       Another confusion with both these properties is that the definition
       is not that the code point has been assigned, but that the meaning
       of the code point has been determined.  This is because 66 code
       points will always be unassigned, and so the "Age" for them is the
       Unicode version in which the decision to make them so was made.
       For example, "U+FDD0" is to be permanently unassigned to a
       character, and the decision to do that was made in version 3.1, so
       "\p{Age=3.1}" matches this character, as also does "\p{Present_In:
       3.1}" and up.

       This matches any character that is graphical or blank, except

       This is the same as "\s", including beyond ASCII.

       Mnemonic: Space, as modified by Perl.  (It doesn't include the
       vertical tab until v5.18, which both the Posix standard and Unicode
       consider white space.)

   "\p{Title}" and  "\p{Titlecase}"
       Under case-sensitive matching, these both match the same code
       points as "\p{General Category=Titlecase_Letter}" ("\p{gc=lt}").
       The difference is that under "/i" caseless matching, these match
       the same as "\p{Cased}", whereas "\p{gc=lt}" matches

       This matches any of the 1_114_112 Unicode code points.  "\p{Any}".

       This is the same as "\v":  A character that changes the spacing

       This is the same as "\w", including over 100_000 characters beyond

       There are several of these, which are the standard Posix classes
       extended to the full Unicode range.  They are described in "POSIX
       Character Classes" in perlrecharclass.

   User-Defined Character Properties
   You can define your own binary character properties by defining
   subroutines whose names begin with "In" or "Is".  (The experimental
   feature "(?[ ])" in perlre provides an alternative which allows more
   complex definitions.)  The subroutines can be defined in any package.
   The user-defined properties can be used in the regular expression
   "\p{}" and "\P{}" constructs; if you are using a user-defined property
   from a package other than the one you are in, you must specify its
   package in the "\p{}" or "\P{}" construct.

       # assuming property Is_Foreign defined in Lang::
       package main;  # property package name required
       if ($txt =~ /\p{Lang::IsForeign}+/) { ... }

       package Lang;  # property package name not required
       if ($txt =~ /\p{IsForeign}+/) { ... }

   Note that the effect is compile-time and immutable once defined.
   However, the subroutines are passed a single parameter, which is 0 if
   case-sensitive matching is in effect and non-zero if caseless matching
   is in effect.  The subroutine may return different values depending on
   the value of the flag, and one set of values will immutably be in
   effect for all case-sensitive matches, and the other set for all case-
   insensitive matches.

   Note that if the regular expression is tainted, then Perl will die
   rather than calling the subroutine when the name of the subroutine is
   determined by the tainted data.

   The subroutines must return a specially-formatted string, with one or
   more newline-separated lines.  Each line must be one of the following:

   *   A single hexadecimal number denoting a code point to include.

   *   Two hexadecimal numbers separated by horizontal whitespace (space
       or tabular characters) denoting a range of code points to include.

   *   Something to include, prefixed by "+": a built-in character
       property (prefixed by "utf8::") or a fully qualified (including
       package name) user-defined character property, to represent all the
       characters in that property; two hexadecimal code points for a
       range; or a single hexadecimal code point.

   *   Something to exclude, prefixed by "-": an existing character
       property (prefixed by "utf8::") or a fully qualified (including
       package name) user-defined character property, to represent all the
       characters in that property; two hexadecimal code points for a
       range; or a single hexadecimal code point.

   *   Something to negate, prefixed "!": an existing character property
       (prefixed by "utf8::") or a fully qualified (including package
       name) user-defined character property, to represent all the
       characters in that property; two hexadecimal code points for a
       range; or a single hexadecimal code point.

   *   Something to intersect with, prefixed by "&": an existing character
       property (prefixed by "utf8::") or a fully qualified (including
       package name) user-defined character property, for all the
       characters except the characters in the property; two hexadecimal
       code points for a range; or a single hexadecimal code point.

   For example, to define a property that covers both the Japanese
   syllabaries (hiragana and katakana), you can define

       sub InKana {
           return <<END;

   Imagine that the here-doc end marker is at the beginning of the line.
   Now you can use "\p{InKana}" and "\P{InKana}".

   You could also have used the existing block property names:

       sub InKana {
           return <<'END';

   Suppose you wanted to match only the allocated characters, not the raw
   block ranges: in other words, you want to remove the unassigned

       sub InKana {
           return <<'END';

   The negation is useful for defining (surprise!) negated classes.

       sub InNotKana {
           return <<'END';

   This will match all non-Unicode code points, since every one of them is
   not in Kana.  You can use intersection to exclude these, if desired, as
   this modified example shows:

       sub InNotKana {
           return <<'END';

   &utf8::Any must be the last line in the definition.

   Intersection is used generally for getting the common characters
   matched by two (or more) classes.  It's important to remember not to
   use "&" for the first set; that would be intersecting with nothing,
   resulting in an empty set.

   Unlike non-user-defined "\p{}" property matches, no warning is ever
   generated if these properties are matched against a non-Unicode code
   point (see "Beyond Unicode code points" below).

   User-Defined Case Mappings (for serious hackers only)
   This feature has been removed as of Perl 5.16.  The CPAN module
   "Unicode::Casing" provides better functionality without the drawbacks
   that this feature had.  If you are using a Perl earlier than 5.16, this
   feature was most fully documented in the 5.14 version of this pod:

   Character Encodings for Input and Output
   See Encode.

   Unicode Regular Expression Support Level
   The following list of Unicode supported features for regular
   expressions describes all features currently directly supported by core
   Perl.  The references to "Level N" and the section numbers refer to the
   Unicode Technical Standard #18, "Unicode Regular Expressions", version
   13, from August 2008.

   *   Level 1 - Basic Unicode Support

        RL1.1   Hex Notation                     - done          [1]
        RL1.2   Properties                       - done          [2][3]
        RL1.2a  Compatibility Properties         - done          [4]
        RL1.3   Subtraction and Intersection     - experimental  [5]
        RL1.4   Simple Word Boundaries           - done          [6]
        RL1.5   Simple Loose Matches             - done          [7]
        RL1.6   Line Boundaries                  - MISSING       [8][9]
        RL1.7   Supplementary Code Points        - done          [10]

       [1] "\N{U+...}" and "\x{...}"
       [2] "\p{...}" "\P{...}"
       [3] supports not only minimal list, but all Unicode character
       properties (see Unicode Character Properties above)
       [4] "\d" "\D" "\s" "\S" "\w" "\W" "\X" "[:prop:]" "[:^prop:]"
       [5] The experimental feature starting in v5.18 "(?[...])"
       accomplishes this.
           See "(?[ ])" in perlre.  If you don't want to use an
           experimental feature, you can use one of the following:

           *   Regular expression lookahead

               You can mimic class subtraction using lookahead.  For
               example, what UTS#18 might write as


               in Perl can be written as:


               But in this particular example, you probably really want


               which will match assigned characters known to be part of
               the Greek script.

           *   CPAN module "Unicode::Regex::Set"

               It does implement the full UTS#18 grouping, intersection,
               union, and removal (subtraction) syntax.

           *   "User-Defined Character Properties"

               "+" for union, "-" for removal (set-difference), "&" for

       [6] "	" "\B"
       [7] Note that Perl does Full case-folding in matching, not Simple:
           For example "U+1F88" is equivalent to "U+1F00 U+03B9", instead
           of just "U+1F80".  This difference matters mainly for certain
           Greek capital letters with certain modifiers: the Full case-
           folding decomposes the letter, while the Simple case-folding
           would map it to a single character.

       [8] Perl treats "\n" as the start- and end-line delimiter.  Unicode
       specifies more characters that should be so-interpreted.
           These are:

            VT   U+000B  (\v in C)
            FF   U+000C  (\f)
            CR   U+000D  (\r)
            NEL  U+0085
            LS   U+2028
            PS   U+2029

           "^" and "$" in regular expression patterns are supposed to
           match all these, but don't.  These characters also don't, but
           should, affect "<>" $., and script line numbers.

           Also, lines should not be split within "CRLF" (i.e. there is no
           empty line between "\r" and "\n").  For "CRLF", try the ":crlf"
           layer (see PerlIO).

       [9] But "qr/	{lb}/" and "Unicode::LineBreak" are available.
           "qr/	{lb}/" supplies default line breaking conformant with
           UAX#14 "Unicode Line Breaking Algorithm"

           And, the module "Unicode::LineBreak" also conformant with
           UAX#14, provides customizable line breaking.

       [10] UTF-8/UTF-EBDDIC used in Perl allows not only "U+10000" to
       "U+10FFFF" but also beyond "U+10FFFF"
   *   Level 2 - Extended Unicode Support

        RL2.1   Canonical Equivalents           - MISSING       [10][11]
        RL2.2   Default Grapheme Clusters       - MISSING       [12]
        RL2.3   Default Word Boundaries         - DONE          [14]
        RL2.4   Default Loose Matches           - MISSING       [15]
        RL2.5   Name Properties                 - DONE
        RL2.6   Wildcard Properties             - MISSING

        [10] see UAX#15 "Unicode Normalization Forms"
        [11] have Unicode::Normalize but not integrated to regexes
        [12] have \X and 	{gcb} but we don't have a "Grapheme Cluster
        [14] see UAX#29, Word Boundaries
        [15] This is covered in Chapter 3.13 (in Unicode 6.0)

   *   Level 3 - Tailored Support

        RL3.1   Tailored Punctuation            - MISSING
        RL3.2   Tailored Grapheme Clusters      - MISSING       [17][18]
        RL3.3   Tailored Word Boundaries        - MISSING
        RL3.4   Tailored Loose Matches          - MISSING
        RL3.5   Tailored Ranges                 - MISSING
        RL3.6   Context Matching                - MISSING       [19]
        RL3.7   Incremental Matches             - MISSING
             ( RL3.8   Unicode Set Sharing )
        RL3.9   Possible Match Sets             - MISSING
        RL3.10  Folded Matching                 - MISSING       [20]
        RL3.11  Submatchers                     - MISSING

        [17] see UAX#10 "Unicode Collation Algorithms"
        [18] have Unicode::Collate but not integrated to regexes
        [19] have (?<=x) and (?=x), but lookaheads or lookbehinds
             should see outside of the target substring
        [20] need insensitive matching for linguistic features other
             than case; for example, hiragana to katakana, wide and
             narrow, simplified Han to traditional Han (see UTR#30
             "Character Foldings")

   Unicode Encodings
   Unicode characters are assigned to code points, which are abstract
   numbers.  To use these numbers, various encodings are needed.

   *   UTF-8

       UTF-8 is a variable-length (1 to 4 bytes), byte-order independent
       encoding.  In most of Perl's documentation, including elsewhere in
       this document, the term "UTF-8" means also "UTF-EBCDIC".  But in
       this section, "UTF-8" refers only to the encoding used on ASCII
       platforms.  It is a superset of 7-bit US-ASCII, so anything encoded
       in ASCII has the identical representation when encoded in UTF-8.

       The following table is from Unicode 3.2.

        Code Points            1st Byte  2nd Byte  3rd Byte 4th Byte

          U+0000..U+007F       00..7F
          U+0080..U+07FF     * C2..DF    80..BF
          U+0800..U+0FFF       E0      * A0..BF    80..BF
          U+1000..U+CFFF       E1..EC    80..BF    80..BF
          U+D000..U+D7FF       ED        80..9F    80..BF
          U+D800..U+DFFF       +++++ utf16 surrogates, not legal utf8 +++++
          U+E000..U+FFFF       EE..EF    80..BF    80..BF
         U+10000..U+3FFFF      F0      * 90..BF    80..BF    80..BF
         U+40000..U+FFFFF      F1..F3    80..BF    80..BF    80..BF
        U+100000..U+10FFFF     F4        80..8F    80..BF    80..BF

       Note the gaps marked by "*" before several of the byte entries
       above.  These are caused by legal UTF-8 avoiding non-shortest
       encodings: it is technically possible to UTF-8-encode a single code
       point in different ways, but that is explicitly forbidden, and the
       shortest possible encoding should always be used (and that is what
       Perl does).

       Another way to look at it is via bits:

                       Code Points  1st Byte  2nd Byte  3rd Byte  4th Byte

                          0aaaaaaa  0aaaaaaa
                  00000bbbbbaaaaaa  110bbbbb  10aaaaaa
                  ccccbbbbbbaaaaaa  1110cccc  10bbbbbb  10aaaaaa
        00000dddccccccbbbbbbaaaaaa  11110ddd  10cccccc  10bbbbbb  10aaaaaa

       As you can see, the continuation bytes all begin with "10", and the
       leading bits of the start byte tell how many bytes there are in the
       encoded character.

       The original UTF-8 specification allowed up to 6 bytes, to allow
       encoding of numbers up to "0x7FFF_FFFF".  Perl continues to allow
       those, and has extended that up to 13 bytes to encode code points
       up to what can fit in a 64-bit word.  However, Perl will warn if
       you output any of these as being non-portable; and under strict
       UTF-8 input protocols, they are forbidden.  In addition, it is
       deprecated to use a code point larger than what a signed integer
       variable on your system can hold.  On 32-bit ASCII systems, this
       means "0x7FFF_FFFF" is the legal maximum going forward (much higher
       on 64-bit systems).


       Like UTF-8, but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
       This means that all the basic characters (which includes all those
       that have ASCII equivalents (like "A", "0", "%", etc.)  are the
       same in both EBCDIC and UTF-EBCDIC.)

       UTF-EBCDIC is used on EBCDIC platforms.  It generally requires more
       bytes to represent a given code point than UTF-8 does; the largest
       Unicode code points take 5 bytes to represent (instead of 4 in
       UTF-8), and, extended for 64-bit words, it uses 14 bytes instead of
       13 bytes in UTF-8.

   *   UTF-16, UTF-16BE, UTF-16LE, Surrogates, and "BOM"'s (Byte Order

       The followings items are mostly for reference and general Unicode
       knowledge, Perl doesn't use these constructs internally.

       Like UTF-8, UTF-16 is a variable-width encoding, but where UTF-8
       uses 8-bit code units, UTF-16 uses 16-bit code units.  All code
       points occupy either 2 or 4 bytes in UTF-16: code points
       "U+0000..U+FFFF" are stored in a single 16-bit unit, and code
       points "U+10000..U+10FFFF" in two 16-bit units.  The latter case is
       using surrogates, the first 16-bit unit being the high surrogate,
       and the second being the low surrogate.

       Surrogates are code points set aside to encode the
       "U+10000..U+10FFFF" range of Unicode code points in pairs of 16-bit
       units.  The high surrogates are the range "U+D800..U+DBFF" and the
       low surrogates are the range "U+DC00..U+DFFF".  The surrogate
       encoding is

           $hi = ($uni - 0x10000) / 0x400 + 0xD800;
           $lo = ($uni - 0x10000) % 0x400 + 0xDC00;

       and the decoding is

           $uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);

       Because of the 16-bitness, UTF-16 is byte-order dependent.  UTF-16
       itself can be used for in-memory computations, but if storage or
       transfer is required either UTF-16BE (big-endian) or UTF-16LE
       (little-endian) encodings must be chosen.

       This introduces another problem: what if you just know that your
       data is UTF-16, but you don't know which endianness?  Byte Order
       Marks, or "BOM"'s, are a solution to this.  A special character has
       been reserved in Unicode to function as a byte order marker: the
       character with the code point "U+FEFF" is the "BOM".

       The trick is that if you read a "BOM", you will know the byte
       order, since if it was written on a big-endian platform, you will
       read the bytes "0xFE 0xFF", but if it was written on a little-
       endian platform, you will read the bytes "0xFF 0xFE".  (And if the
       originating platform was writing in ASCII platform UTF-8, you will
       read the bytes "0xEF 0xBB 0xBF".)

       The way this trick works is that the character with the code point
       "U+FFFE" is not supposed to be in input streams, so the sequence of
       bytes "0xFF 0xFE" is unambiguously ""BOM", represented in little-
       endian format" and cannot be "U+FFFE", represented in big-endian

       Surrogates have no meaning in Unicode outside their use in pairs to
       represent other code points.  However, Perl allows them to be
       represented individually internally, for example by saying
       "chr(0xD801)", so that all code points, not just those valid for
       open interchange, are representable.  Unicode does define semantics
       for them, such as their "General_Category" is "Cs".  But because
       their use is somewhat dangerous, Perl will warn (using the warning
       category "surrogate", which is a sub-category of "utf8") if an
       attempt is made to do things like take the lower case of one, or
       match case-insensitively, or to output them.  (But don't try this
       on Perls before 5.14.)

   *   UTF-32, UTF-32BE, UTF-32LE

       The UTF-32 family is pretty much like the UTF-16 family, except
       that the units are 32-bit, and therefore the surrogate scheme is
       not needed.  UTF-32 is a fixed-width encoding.  The "BOM"
       signatures are "0x00 0x00 0xFE 0xFF" for BE and "0xFF 0xFE 0x00
       0x00" for LE.

   *   UCS-2, UCS-4

       Legacy, fixed-width encodings defined by the ISO 10646 standard.
       UCS-2 is a 16-bit encoding.  Unlike UTF-16, UCS-2 is not extensible
       beyond "U+FFFF", because it does not use surrogates.  UCS-4 is a
       32-bit encoding, functionally identical to UTF-32 (the difference
       being that UCS-4 forbids neither surrogates nor code points larger
       than "0x10_FFFF").

   *   UTF-7

       A seven-bit safe (non-eight-bit) encoding, which is useful if the
       transport or storage is not eight-bit safe.  Defined by RFC 2152.

   Noncharacter code points
   66 code points are set aside in Unicode as "noncharacter code points".
   These all have the "Unassigned" ("Cn") "General_Category", and no
   character will ever be assigned to any of them.  They are the 32 code
   points between "U+FDD0" and "U+FDEF" inclusive, and the 34 code points:

    U+1FFFE  U+1FFFF
    U+2FFFE  U+2FFFF
    U+10FFFE U+10FFFF

   Until Unicode 7.0, the noncharacters were "forbidden for use in open
   interchange of Unicode text data", so that code that processed those
   streams could use these code points as sentinels that could be mixed in
   with character data, and would always be distinguishable from that
   data.  (Emphasis above and in the next paragraph are added in this

   Unicode 7.0 changed the wording so that they are "not recommended for
   use in open interchange of Unicode text data".  The 7.0 Standard goes
   on to say:

       "If a noncharacter is received in open interchange, an application
       is not required to interpret it in any way.  It is good practice,
       however, to recognize it as a noncharacter and to take appropriate
       action, such as replacing it with "U+FFFD" replacement character,
       to indicate the problem in the text.  It is not recommended to
       simply delete noncharacter code points from such text, because of
       the potential security issues caused by deleting uninterpreted
       characters.  (See conformance clause C7 in Section 3.2, Conformance
       Requirements, and Unicode Technical Report #36, "Unicode Security

   This change was made because it was found that various commercial tools
   like editors, or for things like source code control, had been written
   so that they would not handle program files that used these code
   points, effectively precluding their use almost entirely!  And that was
   never the intent.  They've always been meant to be usable within an
   application, or cooperating set of applications, at will.

   If you're writing code, such as an editor, that is supposed to be able
   to handle any Unicode text data, then you shouldn't be using these code
   points yourself, and instead allow them in the input.  If you need
   sentinels, they should instead be something that isn't legal Unicode.
   For UTF-8 data, you can use the bytes 0xC1 and 0xC2 as sentinels, as
   they never appear in well-formed UTF-8.  (There are equivalents for
   UTF-EBCDIC).  You can also store your Unicode code points in integer
   variables and use negative values as sentinels.

   If you're not writing such a tool, then whether you accept
   noncharacters as input is up to you (though the Standard recommends
   that you not).  If you do strict input stream checking with Perl, these
   code points continue to be forbidden.  This is to maintain backward
   compatibility (otherwise potential security holes could open up, as an
   unsuspecting application that was written assuming the noncharacters
   would be filtered out before getting to it, could now, without warning,
   start getting them).  To do strict checking, you can use the layer

   Perl continues to warn (using the warning category "nonchar", which is
   a sub-category of "utf8") if an attempt is made to output

   Beyond Unicode code points
   The maximum Unicode code point is "U+10FFFF", and Unicode only defines
   operations on code points up through that.  But Perl works on code
   points up to the maximum permissible unsigned number available on the
   platform.  However, Perl will not accept these from input streams
   unless lax rules are being used, and will warn (using the warning
   category "non_unicode", which is a sub-category of "utf8") if any are

   Since Unicode rules are not defined on these code points, if a Unicode-
   defined operation is done on them, Perl uses what we believe are
   sensible rules, while generally warning, using the "non_unicode"
   category.  For example, "uc("\x{11_0000}")" will generate such a
   warning, returning the input parameter as its result, since Perl
   defines the uppercase of every non-Unicode code point to be the code
   point itself.  (All the case changing operations, not just uppercasing,
   work this way.)

   The situation with matching Unicode properties in regular expressions,
   the "\p{}" and "\P{}" constructs, against these code points is not as
   clear cut, and how these are handled has changed as we've gained

   One possibility is to treat any match against these code points as
   undefined.  But since Perl doesn't have the concept of a match being
   undefined, it converts this to failing or "FALSE".  This is almost, but
   not quite, what Perl did from v5.14 (when use of these code points
   became generally reliable) through v5.18.  The difference is that Perl
   treated all "\p{}" matches as failing, but all "\P{}" matches as

   One problem with this is that it leads to unexpected, and confusting
   results in some cases:

    chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Failed on <= v5.18
    chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Failed! on <= v5.18

   That is, it treated both matches as undefined, and converted that to
   false (raising a warning on each).  The first case is the expected
   result, but the second is likely counterintuitive: "How could both be
   false when they are complements?"  Another problem was that the
   implementation optimized many Unicode property matches down to already
   existing simpler, faster operations, which don't raise the warning.  We
   chose to not forgo those optimizations, which help the vast majority of
   matches, just to generate a warning for the unlikely event that an
   above-Unicode code point is being matched against.

   As a result of these problems, starting in v5.20, what Perl does is to
   treat non-Unicode code points as just typical unassigned Unicode
   characters, and matches accordingly.  (Note: Unicode has atypical
   unassigned code points.  For example, it has noncharacter code points,
   and ones that, when they do get assigned, are destined to be written
   Right-to-left, as Arabic and Hebrew are.  Perl assumes that no non-
   Unicode code point has any atypical properties.)

   Perl, in most cases, will raise a warning when matching an above-
   Unicode code point against a Unicode property when the result is "TRUE"
   for "\p{}", and "FALSE" for "\P{}".  For example:

    chr(0x110000) =~ \p{ASCII_Hex_Digit=True}      # Fails, no warning
    chr(0x110000) =~ \p{ASCII_Hex_Digit=False}     # Succeeds, with warning

   In both these examples, the character being matched is non-Unicode, so
   Unicode doesn't define how it should match.  It clearly isn't an ASCII
   hex digit, so the first example clearly should fail, and so it does,
   with no warning.  But it is arguable that the second example should
   have an undefined, hence "FALSE", result.  So a warning is raised for

   Thus the warning is raised for many fewer cases than in earlier Perls,
   and only when what the result is could be arguable.  It turns out that
   none of the optimizations made by Perl (or are ever likely to be made)
   cause the warning to be skipped, so it solves both problems of Perl's
   earlier approach.  The most commonly used property that is affected by
   this change is "\p{Unassigned}" which is a short form for
   "\p{General_Category=Unassigned}".  Starting in v5.20, all non-Unicode
   code points are considered "Unassigned".  In earlier releases the
   matches failed because the result was considered undefined.

   The only place where the warning is not raised when it might ought to
   have been is if optimizations cause the whole pattern match to not even
   be attempted.  For example, Perl may figure out that for a string to
   match a certain regular expression pattern, the string has to contain
   the substring "foobar".  Before attempting the match, Perl may look for
   that substring, and if not found, immediately fail the match without
   actually trying it; so no warning gets generated even if the string
   contains an above-Unicode code point.

   This behavior is more "Do what I mean" than in earlier Perls for most
   applications.  But it catches fewer issues for code that needs to be
   strictly Unicode compliant.  Therefore there is an additional mode of
   operation available to accommodate such code.  This mode is enabled if
   a regular expression pattern is compiled within the lexical scope where
   the "non_unicode" warning class has been made fatal, say by:

    use warnings FATAL => "non_unicode"

   (see warnings).  In this mode of operation, Perl will raise the warning
   for all matches against a non-Unicode code point (not just the arguable
   ones), and it skips the optimizations that might cause the warning to
   not be output.  (It currently still won't warn if the match isn't even
   attempted, like in the "foobar" example above.)

   In summary, Perl now normally treats non-Unicode code points as typical
   Unicode unassigned code points for regular expression matches, raising
   a warning only when it is arguable what the result should be.  However,
   if this warning has been made fatal, it isn't skipped.

   There is one exception to all this.  "\p{All}" looks like a Unicode
   property, but it is a Perl extension that is defined to be true for all
   possible code points, Unicode or not, so no warning is ever generated
   when matching this against a non-Unicode code point.  (Prior to v5.20,
   it was an exact synonym for "\p{Any}", matching code points 0 through

   Security Implications of Unicode
   First, read Unicode Security Considerations

   Also, note the following:

   *   Malformed UTF-8

       Unfortunately, the original specification of UTF-8 leaves some room
       for interpretation of how many bytes of encoded output one should
       generate from one input Unicode character.  Strictly speaking, the
       shortest possible sequence of UTF-8 bytes should be generated,
       because otherwise there is potential for an input buffer overflow
       at the receiving end of a UTF-8 connection.  Perl always generates
       the shortest length UTF-8, and with warnings on, Perl will warn
       about non-shortest length UTF-8 along with other malformations,
       such as the surrogates, which are not Unicode code points valid for

   *   Regular expression pattern matching may surprise you if you're not
       accustomed to Unicode.  Starting in Perl 5.14, several pattern
       modifiers are available to control this, called the character set
       modifiers.  Details are given in "Character set modifiers" in

   As discussed elsewhere, Perl has one foot (two hooves?) planted in each
   of two worlds: the old world of ASCII and single-byte locales, and the
   new world of Unicode, upgrading when necessary.  If your legacy code
   does not explicitly use Unicode, no automatic switch-over to Unicode
   should happen.

   Unicode in Perl on EBCDIC
   Unicode is supported on EBCDIC platforms.  See perlebcdic.

   Unless ASCII vs. EBCDIC issues are specifically being discussed,
   references to UTF-8 encoding in this document and elsewhere should be
   read as meaning UTF-EBCDIC on EBCDIC platforms.  See "Unicode and UTF"
   in perlebcdic.

   Because UTF-EBCDIC is so similar to UTF-8, the differences are mostly
   hidden from you; "useutf8" (and NOT something like "useutfebcdic")
   declares the the script is in the platform's "native" 8-bit encoding of
   Unicode.  (Similarly for the ":utf8" layer.)

   See "Unicode and UTF-8" in perllocale

   When Unicode Does Not Happen
   There are still many places where Unicode (in some encoding or another)
   could be given as arguments or received as results, or both in Perl,
   but it is not, in spite of Perl having extensive ways to input and
   output in Unicode, and a few other "entry points" like the @ARGV array
   (which can sometimes be interpreted as UTF-8).

   The following are such interfaces.  Also, see "The "Unicode Bug"".  For
   all of these interfaces Perl currently (as of v5.16.0) simply assumes
   byte strings both as arguments and results, or UTF-8 strings if the
   (deprecated) "encoding" pragma has been used.

   One reason that Perl does not attempt to resolve the role of Unicode in
   these situations is that the answers are highly dependent on the
   operating system and the file system(s).  For example, whether
   filenames can be in Unicode and in exactly what kind of encoding, is
   not exactly a portable concept.  Similarly for "qx" and "system": how
   well will the "command-line interface" (and which of them?) handle

   *   "chdir", "chmod", "chown", "chroot", "exec", "link", "lstat",
       "mkdir", "rename", "rmdir", "stat", "symlink", "truncate",
       "unlink", "utime", "-X"

   *   %ENV

   *   "glob" (aka the "<*>")

   *   "open", "opendir", "sysopen"

   *   "qx" (aka the backtick operator), "system"

   *   "readdir", "readlink"

   The "Unicode Bug"
   The term, "Unicode bug" has been applied to an inconsistency with the
   code points in the "Latin-1 Supplement" block, that is, between 128 and
   255.  Without a locale specified, unlike all other characters or code
   points, these characters can have very different semantics depending on
   the rules in effect.  (Characters whose code points are above 255 force
   Unicode rules; whereas the rules for ASCII characters are the same
   under both ASCII and Unicode rules.)

   Under Unicode rules, these upper-Latin1 characters are interpreted as
   Unicode code points, which means they have the same semantics as
   Latin-1 (ISO-8859-1) and C1 controls.

   As explained in "ASCII Rules versus Unicode Rules", under ASCII rules,
   they are considered to be unassigned characters.

   This can lead to unexpected results.  For example, a string's semantics
   can suddenly change if a code point above 255 is appended to it, which
   changes the rules from ASCII to Unicode.  As an example, consider the
   following program and its output:

    $ perl -le'
        no feature 'unicode_strings';
        $s1 = "\xC2";
        $s2 = "\x{2660}";
        for ($s1, $s2, $s1.$s2) {
            print /\w/ || 0;

   If there's no "\w" in "s1" nor in "s2", why does their concatenation
   have one?

   This anomaly stems from Perl's attempt to not disturb older programs
   that didn't use Unicode, along with Perl's desire to add Unicode
   support seamlessly.  But the result turned out to not be seamless.  (By
   the way, you can choose to be warned when things like this happen.  See

   "usefeature'unicode_strings'" was added, starting in Perl v5.12, to
   address this problem.  It affects these things:

   *   Changing the case of a scalar, that is, using "uc()", "ucfirst()",
       "lc()", and "lcfirst()", or "\L", "\U", "\u" and "\l" in double-
       quotish contexts, such as regular expression substitutions.

       Under "unicode_strings" starting in Perl 5.12.0, Unicode rules are
       generally used.  See "lc" in perlfunc for details on how this works
       in combination with various other pragmas.

   *   Using caseless ("/i") regular expression matching.

       Starting in Perl 5.14.0, regular expressions compiled within the
       scope of "unicode_strings" use Unicode rules even when executed or
       compiled into larger regular expressions outside the scope.

   *   Matching any of several properties in regular expressions.

       These properties are "	" (without braces), "\B" (without braces),
       "\s", "\S", "\w", "\W", and all the Posix character classes except

       Starting in Perl 5.14.0, regular expressions compiled within the
       scope of "unicode_strings" use Unicode rules even when executed or
       compiled into larger regular expressions outside the scope.

   *   In "quotemeta" or its inline equivalent "\Q".

       Starting in Perl 5.16.0, consistent quoting rules are used within
       the scope of "unicode_strings", as described in "quotemeta" in
       perlfunc.  Prior to that, or outside its scope, no code points
       above 127 are quoted in UTF-8 encoded strings, but in byte encoded
       strings, code points between 128-255 are always quoted.

   You can see from the above that the effect of "unicode_strings"
   increased over several Perl releases.  (And Perl's support for Unicode
   continues to improve; it's best to use the latest available release in
   order to get the most complete and accurate results possible.)  Note
   that "unicode_strings" is automatically chosen if you "use5.012" or

   For Perls earlier than those described above, or when a string is
   passed to a function outside the scope of "unicode_strings", see the
   next section.

   Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
   Sometimes (see "When Unicode Does Not Happen" or "The "Unicode Bug"")
   there are situations where you simply need to force a byte string into
   UTF-8, or vice versa.  The standard module Encode can be used for this,
   or the low-level calls "utf8::upgrade($bytestring)" and
   "utf8::downgrade($utf8string[, FAIL_OK])".

   Note that "utf8::downgrade()" can fail if the string contains
   characters that don't fit into a byte.

   Calling either function on a string that already is in the desired
   state is a no-op.

   "ASCII Rules versus Unicode Rules" gives all the ways that a string is
   made to use Unicode rules.

   Using Unicode in XS
   See "Unicode Support" in perlguts for an introduction to Unicode at the
   XS level, and "Unicode Support" in perlapi for the API details.

   Hacking Perl to work on earlier Unicode versions (for very serious hackers
   Perl by default comes with the latest supported Unicode version built-
   in, but the goal is to allow you to change to use any earlier one.  In
   Perls v5.20 and v5.22, however, the earliest usable version is Unicode
   5.1.  Perl v5.18 is able to handle all earlier versions.

   Download the files in the desired version of Unicode from the Unicode
   web site <>).  These should replace the existing
   files in lib/unicore in the Perl source tree.  Follow the instructions
   in README.perl in that directory to change some of their names, and
   then build perl (see INSTALL).

   Porting code from perl-5.6.X
   Perls starting in 5.8 have a different Unicode model from 5.6. In 5.6
   the programmer was required to use the "utf8" pragma to declare that a
   given scope expected to deal with Unicode data and had to make sure
   that only Unicode data were reaching that scope. If you have code that
   is working with 5.6, you will need some of the following adjustments to
   your code. The examples are written such that the code will continue to
   work under 5.6, so you should be safe to try them out.

   *  A filehandle that should read or write UTF-8

        if ($] > 5.008) {
          binmode $fh, ":encoding(utf8)";

   *  A scalar that is going to be passed to some extension

      Be it "Compress::Zlib", "Apache::Request" or any extension that has
      no mention of Unicode in the manpage, you need to make sure that the
      UTF8 flag is stripped off. Note that at the time of this writing
      (January 2012) the mentioned modules are not UTF-8-aware. Please
      check the documentation to verify if this is still true.

        if ($] > 5.008) {
          require Encode;
          $val = Encode::encode_utf8($val); # make octets

   *  A scalar we got back from an extension

      If you believe the scalar comes back as UTF-8, you will most likely
      want the UTF8 flag restored:

        if ($] > 5.008) {
          require Encode;
          $val = Encode::decode_utf8($val);

   *  Same thing, if you are really sure it is UTF-8

        if ($] > 5.008) {
          require Encode;

   *  A wrapper for DBI "fetchrow_array" and "fetchrow_hashref"

      When the database contains only UTF-8, a wrapper function or method
      is a convenient way to replace all your "fetchrow_array" and
      "fetchrow_hashref" calls. A wrapper function will also make it
      easier to adapt to future enhancements in your database driver. Note
      that at the time of this writing (January 2012), the DBI has no
      standardized way to deal with UTF-8 data. Please check the DBI
      documentation to verify if that is still true.

        sub fetchrow {
          # $what is one of fetchrow_{array,hashref}
          my($self, $sth, $what) = @_;
          if ($] < 5.008) {
            return $sth->$what;
          } else {
            require Encode;
            if (wantarray) {
              my @arr = $sth->$what;
              for (@arr) {
                defined && /[^\000-\177]/ && Encode::_utf8_on($_);
              return @arr;
            } else {
              my $ret = $sth->$what;
              if (ref $ret) {
                for my $k (keys %$ret) {
                  && /[^\000-\177]/
                  && Encode::_utf8_on($_) for $ret->{$k};
                return $ret;
              } else {
                defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
                return $ret;

   *  A large scalar that you know can only contain ASCII

      Scalars that contain only ASCII and are marked as UTF-8 are
      sometimes a drag to your program. If you recognize such a situation,
      just remove the UTF8 flag:

        utf8::downgrade($val) if $] > 5.008;


   See also "The "Unicode Bug"" above.

   Interaction with Extensions
   When Perl exchanges data with an extension, the extension should be
   able to understand the UTF8 flag and act accordingly. If the extension
   doesn't recognize that flag, it's likely that the extension will return
   incorrectly-flagged data.

   So if you're working with Unicode data, consult the documentation of
   every module you're using if there are any issues with Unicode data
   exchange. If the documentation does not talk about Unicode at all,
   suspect the worst and probably look at the source to learn how the
   module is implemented. Modules written completely in Perl shouldn't
   cause problems. Modules that directly or indirectly access code written
   in other programming languages are at risk.

   For affected functions, the simple strategy to avoid data corruption is
   to always make the encoding of the exchanged data explicit. Choose an
   encoding that you know the extension can handle. Convert arguments
   passed to the extensions to that encoding and convert results back from
   that encoding. Write wrapper functions that do the conversions for you,
   so you can later change the functions when the extension catches up.

   To provide an example, let's say the popular "Foo::Bar::escape_html"
   function doesn't deal with Unicode data yet. The wrapper function would
   convert the argument to raw UTF-8 and convert the result back to Perl's
   internal representation like so:

       sub my_escape_html ($) {
           my($what) = shift;
           return unless defined $what;

   Sometimes, when the extension does not convert data but just stores and
   retrieves it, you will be able to use the otherwise dangerous
   "Encode::_utf8_on()" function. Let's say the popular "Foo::Bar"
   extension, written in C, provides a "param" method that lets you store
   and retrieve data according to these prototypes:

       $self->param($name, $value);            # set a scalar
       $value = $self->param($name);           # retrieve a scalar

   If it does not yet provide support for any encoding, one could write a
   derived class with such a "param" method:

       sub param {
         my($self,$name,$value) = @_;
         utf8::upgrade($name);     # make sure it is UTF-8 encoded
         if (defined $value) {
           utf8::upgrade($value);  # make sure it is UTF-8 encoded
           return $self->SUPER::param($name,$value);
         } else {
           my $ret = $self->SUPER::param($name);
           Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
           return $ret;

   Some extensions provide filters on data entry/exit points, such as
   "DB_File::filter_store_key" and family. Look out for such filters in
   the documentation of your extensions; they can make the transition to
   Unicode data much easier.

   Some functions are slower when working on UTF-8 encoded strings than on
   byte encoded strings.  All functions that need to hop over characters
   such as "length()", "substr()" or "index()", or matching regular
   expressions can work much faster when the underlying data are byte-

   In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a
   caching scheme was introduced which improved the situation.  In
   general, operations with UTF-8 encoded strings are still slower. As an
   example, the Unicode properties (character classes) like "\p{Nd}" are
   known to be quite a bit slower (5-20 times) than their simpler
   counterparts like "[0-9]" (then again, there are hundreds of Unicode
   characters matching "Nd" compared with the 10 ASCII characters matching


   perlunitut, perluniintro, perluniprops, Encode, open, utf8, bytes,
   perlretut, "${^UNICODE}" in perlvar,


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