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perlunicode



DESCRIPTION

       Important Caveats

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

       Input and Output Layers
           Perl knows when a filehandle uses Perl's internal Uni­
           code 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 partic­
           ular 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.

       Byte and Character Semantics

       Beginning with version 5.6, Perl uses logically-wide char­
       acters 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 char­
       acter semantics for programs.  For operations where Perl
       can unambiguously decide that the input data are charac­
       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 encoun­
       tered by the parser.  Note that this pragma is only
       required while Perl defaults to byte semantics; when char­
       acter 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-Uni­
       code 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.  Other­
       wise, 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 upgraded to 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, so
       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, Uni­
           code 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 recog­
           nized as such and converted to Perl's internal repre­
           sentation only if the appropriate encoding is speci­
           fied.

           Unicode characters can also be added to a string by

       ·   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 charac­
           ters instead of bytes and match against the character
           properties specified in the Unicode properties
           database.  "\w" can be used to match a Japanese ideo­
           graph, for instance.

       ·   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 con­
           sistency 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 proper­
           ties, followed by their long form.  You can use
               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.  "L&" is a special case, which is an
           alias for "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 con­
           cept 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:
               BidiPDF     Pop Directional Format
               BidiEN      European Number
               BidiES      European Number Separator
               BidiET      European Number Terminator
               BidiAN      Arabic Number
               BidiCS      Common Number Separator
               BidiNSM     Non-Spacing Mark
               BidiBN      Boundary Neutral
               BidiB       Paragraph Separator
               BidiS       Segment Separator
               BidiWS      Whitespace
               BidiON      Other Neutrals

           For example, "\p{BidiR}" 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:

           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 proper­
       ties, defined by the PropList Unicode database:

           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 lan­
       guages, while the concept of blocks is more of an artifi­
       cial 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
       flict with a script or any other property, but it is rec­
       ommended that "In" always be used for block tests to avoid
       confusion.

       These block names are supported:

           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
           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 trans­
           lation tables when character input is provided.  Note

       ·   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 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 charac­
           ters are all less than 256 in ordinal value, one
           should not use "~" (the bit complement) with charac­
           ters 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 fol­
           lowing 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


       ·   Something to include, prefixed by "+": a built-in
           character property (prefixed by "utf8::"), to repre­
           sent all the characters in that property; two hexadec­
           imal code points for a range; or a single hexadecimal
           code point.

       ·   Something to exclude, prefixed by "-": an existing
           character property (prefixed by "utf8::"), for 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 charac­
           ter property (prefixed by "utf8::") for all the char­
           acters except the characters in the property; two hex­
           adecimal code points for a range; or a single hexadec­
           imal 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

       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 sub­
       routines in the "main" package with names like "ToLower"
       (for lc() and lcfirst()), "ToTitle" (for the first charac­
       ter 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 map­
       ping is from a single character to another single charac­
       ter, 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
       $Config{privlib}/unicore/To/.  The mapping data is
       returned as the here-document, and the "utf8::ToSpecFoo"
       are special exception mappings derived from <$Con­
       fig{privlib}>/unicore/SpecialCasing.txt.  The "Digit" and
       "Fold" mappings that one can see in the directory are not
       directly user-accessible, one can use either the "Uni­
       code::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 map­
       pings: they will be used only if the scalar has been
       marked as having Unicode characters.  Old byte-style
       sion 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] now scripts (see UTR#24 Script Names) in addition to blocks
                   [ 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.
                        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
           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 encod­
           ings: it is technically possible to UTF-8-encode a
           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 gen­
           eral 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 depen­
           dent.  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 char­
           acter with the code point "U+FEFF" is the BOM.
           in big-endian format".

       ·   UTF-32, UTF-32BE, UTF-32LE

           The UTF-32 family is pretty much like the UTF-16 fam­
           ily, expect that the units are 32-bit, and therefore
           the surrogate scheme is not needed.  The BOM signa­
           tures 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 exten­
           sible beyond "U+FFFF", because it does not use surro­
           gates.  UCS-4 is a 32-bit encoding, functionally iden­
           tical 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
           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 charac­
           ters, 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.
           from bytes to characters when necessary.  If your
           legacy code does not explicitly use Unicode, no auto­
           matic switch-over to characters should happen.  Char­
           acters shouldn't get downgraded to bytes, either.  It
           is possible to accidentally mix bytes and characters,
           however (see perluniintro), in which case "\w" in reg­
           ular 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 per­
       lebcdic 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 stan­
           dard 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 Perl
       currently (as of 5.8.1) simply assumes byte strings both
       as arguments and results.

       One reason why Perl does not attempt to resolve the role

       ·   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($bytestring) and utf8::down­
       grade($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.  Espe­
       cially 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 characters 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 charac­
           ter with code point 0..255 within the string.  Perl's
           Unicode model is not to use UTF-8 until it is abso­
           lutely 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
           used as general-purpose encoding or decoding inter­
           faces: "use Encode" for that.  "sv_utf8_upgrade()" is
           affected by the encoding pragma but "sv_utf8_down­
           grade()" 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 char­
           acters 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 debug­
           ging--they display all characters as hexadecimal code
           points--but with the flags "UNI_DISPLAY_ISPRINT",
           "UNI_DISPLAY_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

       flagged data.

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

       For affected functions, the simple strategy to avoid data
       corruption is to always make the encoding of the exchanged
       data explicit. Choose an encoding that you know the exten­
       sion can handle. Convert arguments passed to the exten­
       sions 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 inter­
       nal 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() func­
       tion. 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 {

       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 Uni­
       code 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

       ·   A wrapper for fetchrow_array and fetchrow_hashref

           When the database contains only UTF-8, a wrapper func­
           tion 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
           recognize such a situation, just remove the UTF-8
           flag:

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


SEE ALSO

       perluniintro, encoding, Encode, open, utf8, bytes, perlre­
       tut, "${^UNICODE}" in perlvar

  
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