MirBSD manpage: perlunicode(1)

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


     perlunicode - Unicode support in Perl


     Important Caveats

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

     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 ":encod-
         ing(...)"  layer.  See open.

         To indicate that Perl source itself is using a particu-
         lar 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 expli-
         cit "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 encod-

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

     "use encoding" needed to upgrade non-Latin-1 byte strings
         By default, there is a fundamental asymmetry in Perl's

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         unicode model: implicit upgrading from byte strings to
         Unicode strings assumes that they were encoded in ISO
         8859-1 (Latin-1), but Unicode strings are downgraded
         with UTF-8 encoding.  This happens because the first 256
         codepoints in Unicode happens to agree with Latin-1.

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

             use encoding 'utf8';

         See "Byte and Character Semantics" for more details.

     Byte and Character Semantics

     Beginning with version 5.6, Perl uses logically-wide charac-
     ters 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 charac-
     ter 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 without additional infor-
     mation from the user, Perl decides in favor of compatibility
     and chooses to use byte semantics.

     This behavior preserves compatibility with earlier versions
     of Perl, which allowed byte semantics in Perl operations
     only if none of the program's inputs were marked as being as
     source of Unicode character data.  Such data may come from
     filehandles, from calls to external programs, from informa-
     tion 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

     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 character seman-
     tics 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

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

     If strings operating under byte semantics and strings with
     Unicode character data are concatenated, the new string will
     be created by decoding the byte strings as ISO 8859-1
     (Latin-1), even if the old 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 character in
     Perl is logically just a number ranging from 0 to 2**31 or
     so. Larger characters may encode into longer sequences of
     bytes internally, but this internal detail is mostly hidden
     for Perl code. See perluniintro for more.

     Effects of Character Semantics

     Character semantics have the following effects:

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

         If you use a Unicode editor to edit your program,
         Unicode 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}".

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     +   If an appropriate encoding is specified, identifiers
         within the Perl script may contain Unicode alphanumeric
         characters, including ideographs.  Perl does not
         currently attempt to canonicalize 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 ideograph, for

         (However, and as a limitation of the current implementa-
         tion, 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 unimpor-
         tant. It is recommended, however, that for consistency
         you use the following naming: the official Unicode
         script, property, or block name (see below for the addi-
         tional rules that apply to block names) with whitespace
         and dashes removed, and the words
         "uppercase-first-lowercase-rest". "Latin-1 Supplement"
         thus becomes "Latin1Supplement".

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

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

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         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

             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

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             C           Other
             Cc          Control
             Cf          Format
             Cs          Surrogate   (not usable)
             Co          PrivateUse
             Cn          Unassigned

         Single-letter properties match all characters in any of
         the two-letter sub-properties starting with the same
         letter. "LC" and "L&" are special cases, which are
         aliases for the set of "Ll", "Lu", and "Lt".

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

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

             Property    Meaning

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

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


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

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     Extended property classes can supplement the basic proper-
     ties, defined by the PropList Unicode database:

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     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}".


     In addition to scripts, Unicode also defines blocks of char-
     acters.  The difference between scripts and blocks is that

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     the concept of scripts is closer to natural languages, while
     the concept of blocks is more of an artificial grouping
     based on groups of 256 Unicode characters. For example, the
     "Latin" script contains letters from many blocks but does
     not contain all the characters from those blocks. It does
     not, for example, contain digits, because digits are shared
     across many scripts. Digits and similar groups, like punc-
     tuation, are in a category called "Common".

     For more about scripts, see the UTR #24:


     For more about blocks, see:


     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:

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     +   The special pattern "\X" matches any extended Unicode
         sequence--"a combining character sequence" in
         Standardese--where the first character is a base charac-
         ter and subsequent characters are mark characters that
         apply to the base character.  "\X" is equivalent to

     +   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 transla-
         tion tables when character input is provided.  Note that
         "uc()", or "\U" in interpolated strings, translates to
         uppercase, while "ucfirst", or "\u" in interpolated
         strings, translates to titlecase in languages that make
         the distinction.

     +   Most operators that deal with positions or lengths in a
         string will automatically switch to using character
         positions, including "chop()", "chomp()", "substr()",
         "pos()", "index()", "rindex()", "sprintf()", "write()",
         and "length()".  Operators that 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.

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     +   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 fol-
         lowing cases:

         +       the case mapping is from a single Unicode char-
                 acter to another single Unicode character, or

         +       the case mapping is from a single Unicode char-
                 acter to more than one Unicode character.

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

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

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

     User-Defined Character Properties

     You can define your own character properties by defining
     subroutines whose names begin with "In" or "Is".  The sub-
     routines 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

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     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 whi-
         tespace (space or tabular characters) denoting a range
         of Unicode code points to include.

     +   Something to include, prefixed by "+": a built-in char-
         acter 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 char-
         acter 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 char-
         acter 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 exist-
         ing character property (prefixed by "utf8::") or a user-
         defined character property, for all the characters
         except the characters in the property; two hexadecimal
         code points for a range; or a single hexadecimal code

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

         sub InKana {
             return <<END;

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

     You could also have used the existing block property names:

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         sub InKana {
             return <<'END';

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

         sub InKana {
             return <<'END';

     The negation is useful for defining (surprise!) negated

         sub InNotKana {
             return <<'END';

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

         sub InFooAndBar {
             return <<'END';

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

     You can also define your own mappings to be used in the
     lc(), lcfirst(), uc(), and ucfirst() (or their string-
     inlined versions). The principle is the same: define subrou-
     tines 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()).

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     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 des-
     tination range. For example:

         sub ToUpper {
             return <<END;

     defines an uc() mapping that causes only the characters "a",
     "b", and "c" to be mapped to "A", "B", "C", all other char-
     acters 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 character,
     leave the end of the source range empty, but the two tabula-
     tor characters are still needed. For example:

         sub ToLower {
             return <<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
     $Config{privlib}/unicore/To/.  The mapping data is returned
     as the here-document, and the "utf8::ToSpecFoo" are special
     exception mappings derived from
     <$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 map-
     pings: 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.

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     Unicode Regular Expression Support Level

     The following list of Unicode support for regular expres-
     sions 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


         in Perl can be written as:


         But in this particular example, you probably really want


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

         Also see the Unicode::Regex::Set module, it does imple-
         ment 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 char-
         acter allocations require 4 bytes), byte-order indepen-
         dent encoding. For ASCII (and we really do mean 7-bit
         ASCII, not another 8-bit encoding), UTF-8 is tran-

         The following table is from Unicode 3.2.

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          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 sin-
         gle 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

     +   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

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         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 computa-
         tions, but if storage or transfer is required either
         UTF-16BE (big-endian) or UTF-16LE (little-endian) encod-
         ings must be chosen.

         This introduces another problem: what if you just know
         that your data is UTF-16, but you don't know which endi-
         anness?  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

         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 for-
         mat" and cannot be "U+FFFE", represented in big-endian

     +   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

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         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-7

         A seven-bit safe (non-eight-bit) encoding, which is use-
         ful 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 char-
         acter.  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 surro-
         gates, 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 dif-
         ferently depending on if data is eight-bit bytes or

         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 perllo-
         cale), the "\w" might contain a few more letters accord-
         ing 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

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         example, your locale might not think that LATIN SMALL
         LETTER ETH is a letter (unless you happen to speak Ice-
         landic), 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 acciden-
         tally mix bytes and characters, however (see perluniin-
         tro), 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 encod-
     ing 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.


     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.

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     The following are such interfaces.  For all of these inter-
     faces 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 depen-
     dent on the operating system and the filesystem(s).  For
     example, whether filenames can be in Unicode, and in exactly
     what kind of encoding, is not exactly a portable concept.
     Similarly for the qx and system: how well will the 'command
     line interface' (and which of them?) handle Unicode?

     +   chdir, chmod, chown, chroot, exec, link, lstat, mkdir,
         rename, rmdir, stat, symlink, truncate, unlink, utime,

     +   %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 sud-
     denly change the 'nature' of scalar like that.  Especially
     careful you have to be if you use the utf8::upgrade(): any
     random byte string is not valid UTF-8.

     Using Unicode in XS

     If you want to handle Perl Unicode in XS extensions, you may
     find the following C APIs useful.  See also "Unicode Sup-
     port" 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

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         there are any characters of code points greater than 255
         (or 127) in the scalar or that there are even any char-
         acters in the scalar.  What the "UTF8" flag means is
         that the sequence of octets in the representation of the
         scalar is the sequence of UTF-8 encoded code points of
         the characters of a string.  The "UTF8" flag being off
         means that each octet in this representation encodes a
         single character with code point 0..255 within the
         string.  Perl's Unicode model is not to use UTF-8 until
         it is absolutely necessary.

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

     +   "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

     +   "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

     +   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 use-
         ful for example for iterating over the characters of a
         UTF-8 encoded buffer; "UNISKIP()" is useful, for exam-
         ple, in computing the size required for a UTF-8 encoded

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     +   "utf8_distance(a, b)" will tell the distance in charac-
         ters 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
         and "UNI_DISPLAY_QQ" you can make the output more read-

     +   "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.


     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 accord-
     ingly. If the extension doesn't know about the flag, it's
     likely that the extension will return incorrectly-flagged

     So if you're working with Unicode data, consult the documen-
     tation 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.

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     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 extensions
     to that encoding and convert results back from that encod-
     ing. Write wrapper functions that do the conversions for
     you, so you can later change the functions when the exten-
     sion catches up.

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

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

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

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

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

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

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


     Some functions are slower when working on UTF-8 encoded
     strings than on byte encoded strings.  All functions that
     need to hop over characters such as length(), substr() or
     index(), or matching regular expressions can work much fas-
     ter 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 hope-
     fully 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 counter-
     parts like "\d" (then again, there 268 Unicode characters
     matching "Nd" compared with the 10 ASCII characters matching

     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 men-
         tioned modules are not UTF-8-aware. Please check the
         documentation to verify if this is still true.

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           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;

     +   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.

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           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;


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

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