perlunicode



PERLUNICODE(1)         Perl Programmers Reference Guide         PERLUNICODE(1)




NAME

       perlunicode - Unicode support in Perl


DESCRIPTION

       Important Caveats

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

       Input and Output Layers
           Perl knows when a filehandle uses Perl’s internal Unicode encodings
           (UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened
           with the ":utf8" layer.  Other encodings can be converted to Perl’s
           encoding on input or from Perl’s encoding on output by use of the
           ":encoding(...)"  layer.  See open.

           To indicate that Perl source itself is using a particular encoding,
           see encoding.

       Regular Expressions
           The regular expression compiler produces polymorphic opcodes.  That
           is, the pattern adapts to the data and automatically switches to
           the Unicode character scheme when presented with Unicode data--or
           instead uses a traditional byte scheme when presented with byte
           data.

       "use utf8" still needed to enable UTF-8/UTF-EBCDIC in scripts
           As a compatibility measure, the "use utf8" pragma must be explic-
           itly included to enable recognition of UTF-8 in the Perl scripts
           themselves (in string or regular expression literals, or in identi-
           fier names) on ASCII-based machines or to recognize UTF-EBCDIC on
           EBCDIC-based machines.  These are the only times when an explicit
           "use utf8" is needed.  See utf8.

           You can also use the "encoding" pragma to change the default encod-
           ing of the data in your script; see encoding.

       BOM-marked scripts and UTF-16 scripts autodetected
           If a Perl script begins marked 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 Unicode.  (BOMless UTF-8 cannot be effectively recognized or
           differentiated from ISO 8859-1 or other eight-bit encodings.)

       "use encoding" needed to upgrade non-Latin-1 byte strings
           By default, there is a fundamental asymmetry in Perl’s unicode
           model: implicit upgrading from byte strings to Unicode strings
           assumes that they were encoded in ISO 8859-1 (Latin-1), but Unicode
           strings are downgraded with UTF-8 encoding.  This happens because
           the first 256 codepoints in Unicode happens to agree with Latin-1.

           If you wish to interpret byte strings as UTF-8 instead, use the
           "encoding" pragma:

               use encoding ’utf8’;

           See "Byte and Character Semantics" for more details.

       Byte and Character Semantics

       Beginning with version 5.6, Perl uses logically-wide characters to
       represent strings internally.

       In future, Perl-level operations will be expected to work with charac-
       ters rather than bytes.

       However, as an interim compatibility measure, Perl aims to provide a
       safe migration path from byte semantics to character semantics for pro-
       grams.  For operations where Perl can unambiguously decide that the
       input data are characters, Perl switches to character semantics.  For
       operations where this determination cannot be made without additional
       information from the user, Perl decides in favor of compatibility and
       chooses to use byte semantics.

       This behavior preserves compatibility with earlier versions of Perl,
       which allowed byte semantics in Perl operations only if none of the
       program’s inputs were marked as being as source of Unicode character
       data.  Such data may come from filehandles, from calls to external pro-
       grams, from information provided by the system (such as %ENV), or from
       literals and constants in the source text.

       The "bytes" pragma will always, regardless of platform, force byte
       semantics in a particular lexical scope.  See bytes.

       The "utf8" pragma is primarily a compatibility device that enables
       recognition of UTF-(8│EBCDIC) in literals encountered by the parser.
       Note that this pragma is only required while Perl defaults to byte
       semantics; when character semantics become the default, this pragma may
       become a no-op.  See utf8.

       Unless explicitly stated, Perl operators use character semantics for
       Unicode data and byte semantics for non-Unicode data.  The decision to
       use character semantics is made transparently.  If input data comes
       from a Unicode source--for example, if a character encoding layer is
       added to a filehandle or a literal Unicode string constant appears in a
       program--character semantics apply.  Otherwise, byte semantics are in
       effect.  The "bytes" pragma should be used to force byte semantics on
       Unicode data.

       If strings operating under byte semantics and strings with Unicode
       character data are concatenated, the new string will be created by
       decoding the byte strings as ISO 8859-1 (Latin-1), even if the old Uni-
       code string used EBCDIC.  This translation is done without regard to
       the system’s native 8-bit encoding.  To change this for systems with
       non-Latin-1 and non-EBCDIC native encodings, use the "encoding" pragma.
       See encoding.

       Under character semantics, many operations that formerly operated on
       bytes now operate on characters. A character in Perl is logically just
       a number ranging from 0 to 2**31 or so. Larger characters may encode
       into longer sequences of bytes internally, but this internal detail is
       mostly hidden for Perl code.  See perluniintro for more.

       Effects of Character Semantics

       Character semantics have the following effects:

       ·   Strings--including hash keys--and regular expression patterns may
           contain characters that have an ordinal value larger than 255.

           If you use a Unicode editor to edit your program, Unicode charac-
           ters may occur directly within the literal strings in one of the
           various Unicode encodings (UTF-8, UTF-EBCDIC, UCS-2, etc.), but
           will be recognized as such and converted to Perl’s internal repre-
           sentation only if the appropriate encoding is specified.

           Unicode characters can also be added to a string by using the
           "\x{...}" notation.  The Unicode code for the desired character, in
           hexadecimal, should be placed in the braces. For instance, a smiley
           face is "\x{263A}".  This encoding scheme only works for characters
           with a code of 0x100 or above.

           Additionally, if you

              use charnames ’:full’;

           you can use the "\N{...}" notation and put the official Unicode
           character name within the braces, such as "\N{WHITE SMILING FACE}".

       ·   If an appropriate encoding is specified, identifiers within the
           Perl script may contain Unicode alphanumeric characters, including
           ideographs.  Perl does not currently attempt to canonicalize vari-
           able names.

       ·   Regular expressions match characters instead of bytes.  "." matches
           a character instead of a byte.  The "\C" pattern is provided to
           force a match a single byte--a "char" in C, hence "\C".

       ·   Character classes in regular expressions match characters instead
           of bytes and match against the character properties specified in
           the Unicode properties database.  "\w" can be used to match a
           Japanese ideograph, for instance.

           (However, and as a limitation of the current implementation, using
           "\w" or "\W" inside a "[...]" character class will still match with
           byte semantics.)

       ·   Named Unicode properties, scripts, and block ranges may be used
           like character classes via the "\p{}" "matches property" construct
           and the  "\P{}" negation, "doesn’t match property".

           For instance, "\p{Lu}" matches any character with the Unicode "Lu"
           (Letter, uppercase) property, while "\p{M}" matches any character
           with an "M" (mark--accents and such) property.  Brackets are not
           required for single letter properties, so "\p{M}" is equivalent to
           "\pM". Many predefined properties are available, such as "\p{Mir-
           rored}" and "\p{Tibetan}".

           The official Unicode script and block names have spaces and dashes
           as separators, but for convenience you can use dashes, spaces, or
           underbars, and case is unimportant. It is recommended, however,
           that for consistency you use the following naming: the official
           Unicode script, property, or block name (see below for the addi-
           tional rules that apply to block names) with whitespace and dashes
           removed, and the words "uppercase-first-lowercase-rest". "Latin-1
           Supplement" thus becomes "Latin1Supplement".

           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}".

           NOTE: the properties, scripts, and blocks listed here are as of
           Unicode 3.2.0, March 2002, or Perl 5.8.0, July 2002.  Unicode 4.0.0
           came out in April 2003, and Perl 5.8.1 in September 2003.

           Here are the basic Unicode General Category properties, followed by
           their long form.  You can use either; "\p{Lu}" and "\p{Uppercase-
           Letter}", for instance, are identical.

               Short       Long

               L           Letter
               LC          CasedLetter
               Lu          UppercaseLetter
               Ll          LowercaseLetter
               Lt          TitlecaseLetter
               Lm          ModifierLetter
               Lo          OtherLetter

               M           Mark
               Mn          NonspacingMark
               Mc          SpacingMark
               Me          EnclosingMark

               N           Number
               Nd          DecimalNumber
               Nl          LetterNumber
               No          OtherNumber

               P           Punctuation
               Pc          ConnectorPunctuation
               Pd          DashPunctuation
               Ps          OpenPunctuation
               Pe          ClosePunctuation
               Pi          InitialPunctuation
                           (may behave like Ps or Pe depending on usage)
               Pf          FinalPunctuation
                           (may behave like Ps or Pe depending on usage)
               Po          OtherPunctuation

               S           Symbol
               Sm          MathSymbol
               Sc          CurrencySymbol
               Sk          ModifierSymbol
               So          OtherSymbol

               Z           Separator
               Zs          SpaceSeparator
               Zl          LineSeparator
               Zp          ParagraphSeparator

               C           Other
               Cc          Control
               Cf          Format
               Cs          Surrogate   (not usable)
               Co          PrivateUse
               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 cases, which are aliases for the set of "Ll", "Lu", and
           "Lt".

           Because Perl hides the need for the user to understand the internal
           representation of Unicode characters, there is no need to implement
           the somewhat messy concept of surrogates. "Cs" is therefore not
           supported.

           Because scripts differ in their directionality--Hebrew is written
           right to left, for example--Unicode supplies these properties in
           the BidiClass class:

               Property    Meaning

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

           For example, "\p{BidiClass:R}" matches characters that are normally
           written right to left.

       Scripts

       The script names which can be used by "\p{...}" and "\P{...}", such as
       in "\p{Latin}" or "\p{Cyrillic}", are as follows:

           Arabic
           Armenian
           Bengali
           Bopomofo
           Buhid
           CanadianAboriginal
           Cherokee
           Cyrillic
           Deseret
           Devanagari
           Ethiopic
           Georgian
           Gothic
           Greek
           Gujarati
           Gurmukhi
           Han
           Hangul
           Hanunoo
           Hebrew
           Hiragana
           Inherited
           Kannada
           Katakana
           Khmer
           Lao
           Latin
           Malayalam
           Mongolian
           Myanmar
           Ogham
           OldItalic
           Oriya
           Runic
           Sinhala
           Syriac
           Tagalog
           Tagbanwa
           Tamil
           Telugu
           Thaana
           Thai
           Tibetan
           Yi

       Extended property classes can supplement the basic properties, defined
       by the PropList Unicode database:

           ASCIIHexDigit
           BidiControl
           Dash
           Deprecated
           Diacritic
           Extender
           GraphemeLink
           HexDigit
           Hyphen
           Ideographic
           IDSBinaryOperator
           IDSTrinaryOperator
           JoinControl
           LogicalOrderException
           NoncharacterCodePoint
           OtherAlphabetic
           OtherDefaultIgnorableCodePoint
           OtherGraphemeExtend
           OtherLowercase
           OtherMath
           OtherUppercase
           QuotationMark
           Radical
           SoftDotted
           TerminalPunctuation
           UnifiedIdeograph
           WhiteSpace

       and there are further derived properties:

           Alphabetic      Lu + Ll + Lt + Lm + Lo + OtherAlphabetic
           Lowercase       Ll + OtherLowercase
           Uppercase       Lu + OtherUppercase
           Math            Sm + OtherMath

           ID_Start        Lu + Ll + Lt + Lm + Lo + Nl
           ID_Continue     ID_Start + Mn + Mc + Nd + Pc

           Any             Any character
           Assigned        Any non-Cn character (i.e. synonym for \P{Cn})
           Unassigned      Synonym for \p{Cn}
           Common          Any character (or unassigned code point)
                           not explicitly assigned to a script

       For backward compatibility (with Perl 5.6), all properties mentioned so
       far may have "Is" prepended to their name, so "\P{IsLu}", for example,
       is equal to "\P{Lu}".

       Blocks

       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 256 Unicode characters. For
       example, the "Latin" script contains letters from many blocks but does
       not contain all the characters from those blocks. It does not, for
       example, contain digits, because digits are shared across many scripts.
       Digits and similar groups, like punctuation, are in a category called
       "Common".

       For more about scripts, see the UTR #24:

          http://www.unicode.org/unicode/reports/tr24/

       For more about blocks, see:

          http://www.unicode.org/Public/UNIDATA/Blocks.txt

       Block names are given with the "In" prefix. For example, the Katakana
       block is referenced via "\p{InKatakana}".  The "In" prefix may be omit-
       ted if there is no naming conflict with a script or any other property,
       but it is recommended that "In" always be used for block tests to avoid
       confusion.

       These block names are supported:

           InAlphabeticPresentationForms
           InArabic
           InArabicPresentationFormsA
           InArabicPresentationFormsB
           InArmenian
           InArrows
           InBasicLatin
           InBengali
           InBlockElements
           InBopomofo
           InBopomofoExtended
           InBoxDrawing
           InBraillePatterns
           InBuhid
           InByzantineMusicalSymbols
           InCJKCompatibility
           InCJKCompatibilityForms
           InCJKCompatibilityIdeographs
           InCJKCompatibilityIdeographsSupplement
           InCJKRadicalsSupplement
           InCJKSymbolsAndPunctuation
           InCJKUnifiedIdeographs
           InCJKUnifiedIdeographsExtensionA
           InCJKUnifiedIdeographsExtensionB
           InCherokee
           InCombiningDiacriticalMarks
           InCombiningDiacriticalMarksforSymbols
           InCombiningHalfMarks
           InControlPictures
           InCurrencySymbols
           InCyrillic
           InCyrillicSupplementary
           InDeseret
           InDevanagari
           InDingbats
           InEnclosedAlphanumerics
           InEnclosedCJKLettersAndMonths
           InEthiopic
           InGeneralPunctuation
           InGeometricShapes
           InGeorgian
           InGothic
           InGreekExtended
           InGreekAndCoptic
           InGujarati
           InGurmukhi
           InHalfwidthAndFullwidthForms
           InHangulCompatibilityJamo
           InHangulJamo
           InHangulSyllables
           InHanunoo
           InHebrew
           InHighPrivateUseSurrogates
           InHighSurrogates
           InHiragana
           InIPAExtensions
           InIdeographicDescriptionCharacters
           InKanbun
           InKangxiRadicals
           InKannada
           InKatakana
           InKatakanaPhoneticExtensions
           InKhmer
           InLao
           InLatin1Supplement
           InLatinExtendedA
           InLatinExtendedAdditional
           InLatinExtendedB
           InLetterlikeSymbols
           InLowSurrogates
           InMalayalam
           InMathematicalAlphanumericSymbols
           InMathematicalOperators
           InMiscellaneousMathematicalSymbolsA
           InMiscellaneousMathematicalSymbolsB
           InMiscellaneousSymbols
           InMiscellaneousTechnical
           InMongolian
           InMusicalSymbols
           InMyanmar
           InNumberForms
           InOgham
           InOldItalic
           InOpticalCharacterRecognition
           InOriya
           InPrivateUseArea
           InRunic
           InSinhala
           InSmallFormVariants
           InSpacingModifierLetters
           InSpecials
           InSuperscriptsAndSubscripts
           InSupplementalArrowsA
           InSupplementalArrowsB
           InSupplementalMathematicalOperators
           InSupplementaryPrivateUseAreaA
           InSupplementaryPrivateUseAreaB
           InSyriac
           InTagalog
           InTagbanwa
           InTags
           InTamil
           InTelugu
           InThaana
           InThai
           InTibetan
           InUnifiedCanadianAboriginalSyllabics
           InVariationSelectors
           InYiRadicals
           InYiSyllables

       ·   The special pattern "\X" matches any extended Unicode sequence--"a
           combining character sequence" in Standardese--where the first char-
           acter is a base character and subsequent characters are mark char-
           acters that apply to the base character.  "\X" is equivalent to
           "(?:\PM\pM*)".

       ·   The "tr///" operator translates characters instead of bytes.  Note
           that the "tr///CU" functionality has been removed.  For similar
           functionality see pack(’U0’, ...) and pack(’C0’, ...).

       ·   Case translation operators use the Unicode case translation tables
           when character input is provided.  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.

       ·   Most operators that deal with positions or lengths in a string will
           automatically switch to using character positions, including
           "chop()", "chomp()", "substr()", "pos()", "index()", "rindex()",
           "sprintf()", "write()", and "length()".  Operators that specifi-
           cally do not switch include "vec()", "pack()", and "unpack()".
           Operators that really don’t care include operators that treats
           strings as a bucket of bits such as "sort()", and operators dealing
           with filenames.

       ·   The "pack()"/"unpack()" letters "c" and "C" do not change, since
           they are often used for byte-oriented formats.  Again, think "char"
           in the C language.

           There is a new "U" specifier that converts between Unicode charac-
           ters and code points.

       ·   The "chr()" and "ord()" functions work on characters, similar to
           "pack("U")" and "unpack("U")", not "pack("C")" and "unpack("C")".
           "pack("C")" and "unpack("C")" are methods for emulating byte-ori-
           ented "chr()" and "ord()" on Unicode strings.  While these methods
           reveal the internal encoding of Unicode strings, that is not some-
           thing one normally needs to care about at all.

       ·   The bit string operators, "& │ ^ ~", can operate on character data.
           However, for backward compatibility, such as when using bit string
           operations when characters are all less than 256 in ordinal value,
           one should not use "~" (the bit complement) with characters of both
           values less than 256 and values greater than 256.  Most impor-
           tantly, DeMorgan’s laws ("~($x│$y) eq ~$x&~$y" and "~($x&$y) eq
           ~$x│~$y") will not hold.  The reason for this mathematical faux pas
           is that the complement cannot return both the 8-bit (byte-wide) bit
           complement and the full character-wide bit complement.

       ·   lc(), uc(), lcfirst(), and ucfirst() work for the following cases:

           ·       the case mapping is from a single Unicode character to
                   another single Unicode character, or

           ·       the case mapping is from a single Unicode character to more
                   than one Unicode character.

           Things to do with locales (Lithuanian, Turkish, Azeri) do not work
           since Perl does not understand the concept of Unicode locales.

           See the Unicode Technical Report #21, Case Mappings, for more
           details.

       ·   And finally, "scalar reverse()" reverses by character rather than
           by byte.

       User-Defined Character Properties

       You can define your own character properties by defining subroutines
       whose names begin with "In" or "Is".  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 IsForeign 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.

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

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

       ·   Something to include, prefixed by "+": a built-in character prop-
           erty (prefixed by "utf8::") or a 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 prop-
           erty (prefixed by "utf8::") or a 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 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 user-defined character prop-
           erty, 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 syl-
       labaries (hiragana and katakana), you can define

           sub InKana {
               return <<END;
           3040\t309F
           30A0\t30FF
           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’;
           +utf8::InHiragana
           +utf8::InKatakana
           END
           }

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

           sub InKana {
               return <<’END’;
           +utf8::InHiragana
           +utf8::InKatakana
           -utf8::IsCn
           END
           }

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

           sub InNotKana {
               return <<’END’;
           !utf8::InHiragana
           -utf8::InKatakana
           +utf8::IsCn
           END
           }

       Intersection is useful for getting the common characters matched by two
       (or more) classes.

           sub InFooAndBar {
               return <<’END’;
           +main::Foo
           &main::Bar
           END
           }

       It’s important to remember not to use "&" for the first set -- that
       would be intersecting with nothing (resulting in an empty set).

       You can also define your own mappings to be used in the lc(),
       lcfirst(), uc(), and ucfirst() (or their string-inlined versions).  The
       principle is the same: define subroutines in the "main" package with
       names like "ToLower" (for lc() and lcfirst()), "ToTitle" (for the first
       character in ucfirst()), and "ToUpper" (for uc(), and the rest of the
       characters in ucfirst()).

       The string returned by the subroutines needs now to be three hexadeci-
       mal numbers separated by tabulators: start of the source range, end of
       the source range, and start of the destination range.  For example:

           sub ToUpper {
               return <<END;
           0061\t0063\t0041
           END
           }

       defines an uc() mapping that causes only the characters "a", "b", and
       "c" to be mapped to "A", "B", "C", all other characters will remain
       unchanged.

       If there is no source range to speak of, that is, the mapping is from a
       single character to another single character, leave the end of the
       source range empty, but the two tabulator characters are still needed.
       For example:

           sub ToLower {
               return <<END;
           0041\t\t0061
           END
           }

       defines a lc() mapping that causes only "A" to be mapped to "a", all
       other characters will remain unchanged.

       (For serious hackers only)  If you want to introspect the default
       mappings, you can find the data in the directory $Config{privlib}/uni-
       core/To/.  The mapping data is returned as the here-document, and the
       "utf8::ToSpecFoo" are special exception mappings derived from <$Con-
       fig{privlib}>/unicore/SpecialCasing.txt.  The "Digit" and "Fold" map-
       pings that one can see in the directory are not directly user-accessi-
       ble, one can use either the "Unicode::UCD" module, or just match case-
       insensitively (that’s when the "Fold" mapping is used).

       A final note on the user-defined property tests and mappings: they will
       be used only if the scalar has been marked as having Unicode charac-
       ters.  Old byte-style strings will not be affected.

       Character Encodings for Input and Output

       See Encode.

       Unicode Regular Expression Support Level

       The following list of Unicode support for regular expressions describes
       all the features currently supported.  The references to "Level N" and
       the section numbers refer to the Unicode Technical Report 18, "Unicode
       Regular Expression Guidelines", version 6 (Unicode 3.2.0, Perl 5.8.0).

       ·   Level 1 - Basic Unicode Support

                   2.1 Hex Notation                        - done          [1]
                       Named Notation                      - done          [2]
                   2.2 Categories                          - done          [3][4]
                   2.3 Subtraction                         - MISSING       [5][6]
                   2.4 Simple Word Boundaries              - done          [7]
                   2.5 Simple Loose Matches                - done          [8]
                   2.6 End of Line                         - MISSING       [9][10]

                   [ 1] \x{...}
                   [ 2] \N{...}
                   [ 3] . \p{...} \P{...}
                   [ 4] support for scripts (see UTR#24 Script Names), blocks,
                        binary properties, enumerated non-binary properties, and
                        numeric properties (as listed in UTR#18 Other Properties)
                   [ 5] have negation
                   [ 6] can use regular expression look-ahead [a]
                        or user-defined character properties [b] to emulate subtraction
                   [ 7] include Letters in word characters
                   [ 8] note that Perl does Full case-folding in matching, not Simple:
                        for example U+1F88 is equivalent with U+1F00 U+03B9,
                        not with 1F80.  This difference matters 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.
                   [ 9] see UTR #13 Unicode Newline Guidelines
                   [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029}
                        (should also affect <>, $., and script line numbers)
                        (the \x{85}, \x{2028} and \x{2029} do match \s)

           [a] You can mimic class subtraction using lookahead.  For example,
           what UTR #18 might write as

               [{Greek}-[{UNASSIGNED}]]

           in Perl can be written as:

               (?!\p{Unassigned})\p{InGreekAndCoptic}
               (?=\p{Assigned})\p{InGreekAndCoptic}

           But in this particular example, you probably really want

               \p{GreekAndCoptic}

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

           Also see the Unicode::Regex::Set module, it does implement the full
           UTR #18 grouping, intersection, union, and removal (subtraction)
           syntax.

           [b] See "User-Defined Character Properties".

       ·   Level 2 - Extended Unicode Support

                   3.1 Surrogates                          - MISSING       [11]
                   3.2 Canonical Equivalents               - MISSING       [12][13]
                   3.3 Locale-Independent Graphemes        - MISSING       [14]
                   3.4 Locale-Independent Words            - MISSING       [15]
                   3.5 Locale-Independent Loose Matches    - MISSING       [16]

                   [11] Surrogates are solely a UTF-16 concept and Perl’s internal
                        representation is UTF-8.  The Encode module does UTF-16, though.
                   [12] see UTR#15 Unicode Normalization
                   [13] have Unicode::Normalize but not integrated to regexes
                   [14] have \X but at this level . should equal that
                   [15] need three classes, not just \w and \W
                   [16] see UTR#21 Case Mappings

       ·   Level 3 - Locale-Sensitive Support

                   4.1 Locale-Dependent Categories         - MISSING
                   4.2 Locale-Dependent Graphemes          - MISSING       [16][17]
                   4.3 Locale-Dependent Words              - MISSING
                   4.4 Locale-Dependent Loose Matches      - MISSING
                   4.5 Locale-Dependent Ranges             - MISSING

                   [16] see UTR#10 Unicode Collation Algorithms
                   [17] have Unicode::Collate but not integrated to regexes

       Unicode Encodings

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

       ·   UTF-8

           UTF-8 is a variable-length (1 to 6 bytes, current character alloca-
           tions require 4 bytes), byte-order independent encoding. For ASCII
           (and we really do mean 7-bit ASCII, not another 8-bit encoding),
           UTF-8 is transparent.

           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       ******* ill-formed *******
              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 "A0..BF" in "U+0800..U+0FFF", the "80..9F" in
           "U+D000...U+D7FF", the "90..B"F in "U+10000..U+3FFFF", and the
           "80...8F" in "U+100000..U+10FFFF".  The "gaps" 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.  So that’s 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 the are in the
           encoded character.

       ·   UTF-EBCDIC

           Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.

       ·   UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)

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

           UTF-16 is a 2 or 4 byte encoding.  The Unicode code points
           "U+0000..U+FFFF" are stored in a single 16-bit unit, and the 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);

           If you try to generate surrogates (for example by using chr()), you
           will get a warning if warnings are turned on, because those code
           points are not valid for a Unicode character.

           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 (lit-
           tle-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 BOMs, 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 origi-
           nating platform was writing in 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 guaranteed not to be a valid Unicode character, so the
           sequence of bytes "0xFF 0xFE" is unambiguously "BOM, represented in
           little-endian format" and cannot be "U+FFFE", represented in big-
           endian format".

       ·   UTF-32, UTF-32BE, UTF-32LE

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

       ·   UCS-2, UCS-4

           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.

       ·   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.

       Security Implications of Unicode


       ·   Malformed UTF-8

           Unfortunately, the specification of UTF-8 leaves some room for
           interpretation of how many bytes of encoded output one should gen-
           erate 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 real Unicode code points.

       ·   Regular expressions behave slightly differently between byte data
           and character (Unicode) data.  For example, the "word character"
           character class "\w" will work differently depending on if data is
           eight-bit bytes or Unicode.

           In the first case, the set of "\w" characters is either small--the
           default set of alphabetic characters, digits, and the "_"--or, if
           you are using a locale (see perllocale), the "\w" might contain a
           few more letters according to your language and country.

           In the second case, the "\w" set of characters is much, much
           larger.  Most importantly, even in the set of the first 256 charac-
           ters, it will probably match different characters: unlike most
           locales, which are specific to a language and country pair, Unicode
           classifies all the characters that are letters somewhere as "\w".
           For example, your locale might not think that LATIN SMALL LETTER
           ETH is a letter (unless you happen to speak Icelandic), but Unicode
           does.

           As discussed elsewhere, Perl has one foot (two hooves?) planted in
           each of two worlds: the old world of bytes and the new world of
           characters, upgrading from bytes to characters when necessary.  If
           your legacy code does not explicitly use Unicode, no automatic
           switch-over to characters should happen.  Characters shouldn’t get
           downgraded to bytes, either.  It is possible to accidentally mix
           bytes and characters, however (see perluniintro), in which case
           "\w" in regular expressions might start behaving differently.
           Review your code.  Use warnings and the "strict" pragma.

       Unicode in Perl on EBCDIC

       The way Unicode is handled on EBCDIC platforms is still experimental.
       On such platforms, references to UTF-8 encoding in this document and
       elsewhere should be read as meaning the UTF-EBCDIC specified in Unicode
       Technical Report 16, unless ASCII vs. EBCDIC issues are specifically
       discussed. There is no "utfebcdic" pragma or ":utfebcdic" layer;
       rather, "utf8" and ":utf8" are reused to mean the platform’s "natural"
       8-bit encoding of Unicode. See perlebcdic for more discussion of the
       issues.

       Locales

       Usually locale settings and Unicode do not affect each other, but there
       are a couple of exceptions:

       ·   You can enable automatic UTF-8-ification of your standard file han-
           dles, default "open()" layer, and @ARGV by using either the "-C"
           command line switch or the "PERL_UNICODE" environment variable, see
           perlrun for the documentation of the "-C" switch.

       ·   Perl tries really hard to work both with Unicode and the old byte-
           oriented world. Most often this is nice, but sometimes Perl’s
           straddling of the proverbial fence causes problems.

       When Unicode Does Not Happen

       While Perl does have extensive ways to input and output in Unicode, and
       few other ’entry points’ like the @ARGV which can be interpreted as
       Unicode (UTF-8), there still are many places where Unicode (in some
       encoding or another) could be given as arguments or received as
       results, or both, but it is not.

       The following are such interfaces.  For all of these interfaces Perl
       currently (as of 5.8.3) simply assumes byte strings both as arguments
       and results, or UTF-8 strings if the "encoding" pragma has been used.

       One reason why Perl does not attempt to resolve the role of Unicode in
       this cases 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 the qx and system: how well will the
       ’command line interface’ (and which of them?) handle Unicode?

       ·   chmod, 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

       Forcing Unicode in Perl (Or Unforcing Unicode in Perl)

       Sometimes (see "When Unicode Does Not Happen") there are situations
       where you simply need to force Perl to believe that a byte string is
       UTF-8, or vice versa.  The low-level calls utf8::upgrade($bytestring)
       and utf8::downgrade($utf8string) are the answers.

       Do not use them without careful thought, though: Perl may easily get
       very confused, angry, or even crash, if you suddenly change the
       ’nature’ of scalar like that.  Especially careful you have to be if you
       use the utf8::upgrade(): any random byte string is not valid UTF-8.

       Using Unicode in XS

       If you want to handle Perl Unicode in XS extensions, you may find the
       following C APIs useful.  See also "Unicode Support" in perlguts for an
       explanation about Unicode at the XS level, and perlapi for the API
       details.

       ·   "DO_UTF8(sv)" returns true if the "UTF8" flag is on and the bytes
           pragma is not in effect.  "SvUTF8(sv)" returns true is the "UTF8"
           flag is on; the bytes pragma is ignored.  The "UTF8" flag being on
           does not mean that there are any characters of code points greater
           than 255 (or 127) in the scalar or that there are even any charac-
           ters in the scalar.  What the "UTF8" flag means is that the
           sequence of octets in the representation of the scalar is the
           sequence of UTF-8 encoded code points of the characters of a
           string.  The "UTF8" flag being off means that each octet in this
           representation encodes a single character with code point 0..255
           within the string.  Perl’s Unicode model is not to use UTF-8 until
           it is absolutely necessary.

       ·   "uvuni_to_utf8(buf, chr)" writes a Unicode character code point
           into a buffer encoding the code point as UTF-8, and returns a
           pointer pointing after the UTF-8 bytes.

       ·   "utf8_to_uvuni(buf, lenp)" reads UTF-8 encoded bytes from a buffer
           and returns the Unicode character code point and, optionally, the
           length of the UTF-8 byte sequence.

       ·   "utf8_length(start, end)" returns the length of the UTF-8 encoded
           buffer in characters.  "sv_len_utf8(sv)" returns the length of the
           UTF-8 encoded scalar.

       ·   "sv_utf8_upgrade(sv)" converts the string of the scalar to its
           UTF-8 encoded form.  "sv_utf8_downgrade(sv)" does the opposite, if
           possible.  "sv_utf8_encode(sv)" is like sv_utf8_upgrade except that
           it does not set the "UTF8" flag.  "sv_utf8_decode()" does the oppo-
           site of "sv_utf8_encode()".  Note that none of these are to be used
           as general-purpose encoding or decoding interfaces: "use Encode"
           for that.  "sv_utf8_upgrade()" is affected by the encoding pragma
           but "sv_utf8_downgrade()" is not (since the encoding pragma is
           designed to be a one-way street).

       ·   is_utf8_char(s) returns true if the pointer points to a valid UTF-8
           character.

       ·   "is_utf8_string(buf, len)" returns true if "len" bytes of the
           buffer are valid UTF-8.

       ·   "UTF8SKIP(buf)" will return the number of bytes in the UTF-8
           encoded character in the buffer.  "UNISKIP(chr)" will return the
           number of bytes required to UTF-8-encode the Unicode character code
           point.  "UTF8SKIP()" is useful for example for iterating over the
           characters of a UTF-8 encoded buffer; "UNISKIP()" is useful, for
           example, in computing the size required for a UTF-8 encoded buffer.

       ·   "utf8_distance(a, b)" will tell the distance in characters between
           the two pointers pointing to the same UTF-8 encoded buffer.

       ·   "utf8_hop(s, off)" will return a pointer to an UTF-8 encoded buffer
           that is "off" (positive or negative) Unicode characters displaced
           from the UTF-8 buffer "s".  Be careful not to overstep the buffer:
           "utf8_hop()" will merrily run off the end or the beginning of the
           buffer if told to do so.

       ·   "pv_uni_display(dsv, spv, len, pvlim, flags)" and "sv_uni_dis-
           play(dsv, ssv, pvlim, flags)" are useful for debugging the output
           of Unicode strings and scalars.  By default they are useful only
           for debugging--they display all characters as hexadecimal code
           points--but with the flags "UNI_DISPLAY_ISPRINT", "UNI_DIS-
           PLAY_BACKSLASH", and "UNI_DISPLAY_QQ" you can make the output more
           readable.

       ·   "ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2)" can be used to
           compare two strings case-insensitively in Unicode.  For case-sensi-
           tive comparisons you can just use "memEQ()" and "memNE()" as usual.

       For more information, see perlapi, and utf8.c and utf8.h in the Perl
       source code distribution.


BUGS

       Interaction with Locales

       Use of locales with Unicode data may lead to odd results.  Currently,
       Perl attempts to attach 8-bit locale info to characters in the range
       0..255, but this technique is demonstrably incorrect for locales that
       use characters above that range when mapped into Unicode.  Perl’s Uni-
       code support will also tend to run slower.  Use of locales with Unicode
       is discouraged.

       Interaction with Extensions

       When Perl exchanges data with an extension, the extension should be
       able to understand the UTF-8 flag and act accordingly. If the extension
       doesn’t know about the 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, sus-
       pect 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 $what;
             Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
           }

       Sometimes, when the extension does not convert data but just stores and
       retrieves them, you will be in a position 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 Uni-
       code data much easier.

       Speed

       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-encoded.

       In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a
       caching scheme was introduced which will hopefully make the slowness
       somewhat less spectacular, at least for some operations.  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
       "\d" (then again, there 268 Unicode characters matching "Nd" compared
       with the 10 ASCII characters matching "d").

       Porting code from perl-5.6.X

       Perl 5.8 has 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 Uni-
       code 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 ($] > 5.007) {
               binmode $fh, ":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
           UTF-8 flag is stripped off. Note that at the time of this writing
           (October 2002) the mentioned modules are not UTF-8-aware. Please
           check the documentation to verify if this is still true.

             if ($] > 5.007) {
               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 UTF-8 flag restored:

             if ($] > 5.007) {
               require Encode;
               $val = Encode::decode_utf8($val);
             }

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

             if ($] > 5.007) {
               require Encode;
               Encode::_utf8_on($val);
             }

       ·   A wrapper for 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 (October 2002), the DBI has no stan-
           dardized way to deal with UTF-8 data. Please check the documenta-
           tion to verify if that is still true.

             sub fetchrow {
               my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref}
               if ($] < 5.007) {
                 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) {
                       defined && /[^\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 some-
           times a drag to your program. If you recognize such a situation,
           just remove the UTF-8 flag:

             utf8::downgrade($val) if $] > 5.007;


SEE ALSO

       perluniintro, encoding, Encode, open, utf8, bytes, perlretut, "${^UNI-
       CODE}" in perlvar



perl v5.8.6                       2004-11-05                    PERLUNICODE(1)

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