Man Pages

perlunicode(1) - phpMan perlunicode(1) - phpMan

Command: man perldoc info search(apropos)  


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 implement the Unicode standard or the accompa-
       nying 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 explicitly included to enable recognition of
           UTF-8 in the Perl scripts themselves (in string or regular expression literals, or in identifier 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 encoding 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.  (BOM-
           less UTF-8 cannot be effectively recognized or differentiated from ISO 8859-1 or other eight-bit encod-
           ings.)

       "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 down-
           graded 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 characters rather than bytes.

       However, as an interim compatibility measure, Perl aims to provide a safe migration path from byte semantics to
       character semantics for programs.  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 with-
       out additional information from the user, Perl decides in favor of compatibility and chooses to use byte seman-
       tics.

       This behavior preserves compatibility with earlier versions of Perl, which allowed byte semantics in Perl oper-
       ations 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 programs, 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 Uni-
       code 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 Unicode 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 charac-
       ter 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 characters 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 representation 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 alphanu-
           meric characters, including ideographs.  Perl does not currently attempt to canonicalize variable 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{Mirrored}" 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 consis-
           tency you use the following naming: the official Unicode script, property, or block name (see below for the
           additional rules that apply to block names) with whitespace and dashes removed, and the words "upper-
           case-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{UppercaseLetter}", 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 sup-
           plies 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 arti-
       ficial 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 omitted 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 Stan-
           dardese--where the first character is a base character and subsequent characters are mark characters 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 interpo-
           lated 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 specifically 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 for-
           mats.  Again, think "char" in the C language.

           There is a new "U" specifier that converts between Unicode characters 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-oriented
           "chr()" and "ord()" on Unicode strings.  While these methods reveal the internal encoding of Unicode
           strings, that is not something 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 importantly, 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 con-
           cept 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 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 exclude, prefixed by "-": 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 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 property, for all the characters except the characters in the property; two hexadec-
           imal 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;
           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 (result-
       ing 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 hexadecimal 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 direc-
       tory $Config{privlib}/unicore/To/.  The mapping data is returned as the here-document, and the "utf8::ToSpec-
       Foo" are special exception mappings derived from <$Config{privlib}>/unicore/SpecialCasing.txt.  The "Digit" and
       "Fold" mappings that one can see in the directory are not directly user-accessible, 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 characters.  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 numbers.  To use these numbers, various
       encodings are needed.

       ?   UTF-8

           UTF-8 is a variable-length (1 to 6 bytes, current character allocations require 4 bytes), byte-order inde-
           pendent 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 avoid-
           ing 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 con-
           structs 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 compu-
           tations, 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 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 originating 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 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 over-
           flow 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 sur-
           rogates, which are not real Unicode code points.

       ?   Regular expressions behave slightly differently between byte data and character (Unicode) data.  For exam-
           ple, 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 let-
           ters 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 characters, 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 hap-
           pen 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, how-
           ever (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 handles, 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 Uni-
       code, and in exactly what kind of encoding, is not exactly a portable concept.  Similarly for the qx and sys-
       tem: how well will the 'command line interface' (and which of them?) handle Unicode?

       ?   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

       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 "Uni-
       code 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 characters in the scalar.  What the "UTF8" flag means is that the sequence of octets in the representa-
           tion 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 opposite 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_display(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-sensitive 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 Unicode 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 incor-
       rectly-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 lan-
       guages 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 posi-
       tion 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.

       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, opera-
       tions 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 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 ($] > 5.007) {
               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
           standardized way to deal with UTF-8 data. Please check the documentation 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 sometimes a drag to your program. If you recog-
           nize such a situation, just remove the UTF-8 flag:

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

SEE ALSO
       perluniintro, encoding, Encode, open, utf8, bytes, perlretut, "${^UNICODE}" in perlvar



perl v5.8.8                       2006-01-07                    PERLUNICODE(1)