*nix Documentation Project
·  Home
 +   man pages
·  Linux HOWTOs
·  FreeBSD Tips
·  *niX Forums

  man pages->OpenBSD man pages -> perlunicode (1)              



NAME    [Toc]    [Back]

       perlunicode - Unicode support in Perl

DESCRIPTION    [Toc]    [Back]

       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
           ":encoding(...)"  layer.  See open.

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

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

       "use utf8" still needed to enable UTF-8/UTF-EBCDIC in
           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 ASCIIbased
 machines or to recognize UTF-EBCDIC on EBCDICbased
 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

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

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

           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    [Toc]    [Back]

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

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

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

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

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

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

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

       If strings operating under byte semantics and strings with
       Unicode character data are concatenated, the new string
       will be created by decoding the byte strings as ISO 8859-1
       (Latin-1), even if the old 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    [Toc]    [Back]

       Character semantics have the following effects:

       o   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 ".}" notation.  The Unicode code for
           the desired character, in hexadecimal, should be
           placed in the braces. For instance, a smiley face is
           "}".  This encoding scheme only works for char

           acters with a code of 0x100 or above.

           Additionally, if you

              use charnames ':full';

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

       o   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.

       o   Regular expressions match characters instead of bytes.
           "."  matches  a  character  instead  of a byte.  The "
pattern is provided to force a match a single byte--a
           "char" in C, hence "
       o   Character classes in regular expressions match characters
 instead of bytes and match against the character
           properties specified in the Unicode properties
           database.  "912
           graph, for instance.

           (However, and as a limitation of the current implementation,
 using "96W" inside a "[...]" character
           class will still match with byte semantics.)

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

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

           The official Unicode script and block names have
           spaces and dashes as separators, but for convenience
           you can use dashes, spaces, or underbars, and case is
           unimportant. It is recommended, however, that for consistency
 you use the following naming: the official
           Unicode script, property, or block name (see below for
           the additional 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   "{}"
and "P{}" by
           introducing a caret (^) between the first brace and
           the         property         name:          "{^Tamil}"
is equal to

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

           Here are the basic Unicode General Category
           properties, followed by their long form.  You can use
           either;                                         "{Lu}"
and                                          "{UppercaseLetter}",
           instance, are identical.

               Short       Long

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

               M           Mark
               Mn          NonspacingMark
               Mc          SpacingMark
               Me          EnclosingMark

               N           Number
               Nd          DecimalNumber
               Nl          LetterNumber
               No          OtherNumber

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

               S           Symbol
               Sm          MathSymbol
               Sc          CurrencySymbol
               Sk          ModifierSymbol
               So          OtherSymbol

               Z           Separator
               Zs          SpaceSeparator
               Zl          LineSeparator
               Zp          ParagraphSeparator

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

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

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

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

               Property    Meaning

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

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

       Scripts    [Toc]    [Back]

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

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

       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
           Unassigned             Synonym        for         {Cn}
           Common           Any  character  (or  unassigned  code
                           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    [Toc]    [Back]

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

       For more about scripts, see the UTR #24:


       For more about blocks, see:


       Block names are given with the "In" prefix. For example,
       the  Katakana  block  is  referenced  via  "{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

       These block names are supported:

       o   The special pattern "
           sequence--"a combining character sequence" in Standardese--where
 the first character is a base character
           and subsequent characters are mark characters that
           apply to the base character.  "

       o   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', ...).

       o   Case translation operators use the Unicode case translation
 tables when character input is provided.  Note
           that "uc()", or "U" in interpolated strings, translates
 to uppercase, while "ucfirst", or "" in interpolated
 strings, translates to titlecase in languages
           that make the distinction.

       o   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.

       o   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.

       o   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.

       o   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.

       o   lc(), uc(), lcfirst(), and ucfirst() work for the following

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

           o       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.

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

       User-Defined Character Properties    [Toc]    [Back]

       You can define your own character properties by defining
       subroutines whose names begin with "In" or "Is".  The subroutines
 can be defined in any package.  The user-defined
       properties  can  be  used  in  the  regular  expression ""
       "P" 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       ""
or "P" construct.

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

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

       Note that the effect is compile-time and immutable once

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

       o   Two hexadecimal numbers separated by horizontal
           whitespace (space or tabular characters) denoting a
           range of Unicode code points to include.
       o   Something to include, prefixed by "+": a built-in
           character property (prefixed by "utf8::") or a userdefined
 character property, to represent all the characters
 in that property; two hexadecimal code points
           for a range; or a single hexadecimal code point.

       o   Something to exclude, prefixed by "-": an existing
           character property (prefixed by "utf8::") or a userdefined
 character property, to represent all the characters
 in that property; two hexadecimal code points
           for a range; or a single hexadecimal code point.

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

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

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

           sub InKana {
               return <<END;

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

       You could also have used the existing block property

           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 stringinlined
 versions).  The principle is the same: define subroutines
 in the "main" package with names like "ToLower"
       (for lc() and lcfirst()), "ToTitle" (for the first character
 in ucfirst()), and "ToUpper" (for uc(), and the rest
       of the characters in ucfirst()).

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

           sub ToUpper {
               return <<END;

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

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

           sub ToLower {
               return <<END;

       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 mappings:
 they will be used only if the scalar has been
       marked as having Unicode characters.  Old byte-style
       strings will not be affected.

       Character Encodings for Input and Output    [Toc]    [Back]

       See Encode.

       Unicode Regular Expression Support Level    [Toc]    [Back]

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

       o   Level 1 - Basic Unicode Support

                   2.1 Hex Notation                        - done
                       Named Notation                      - done
                   2.2 Categories                          - done
                   2.3   Subtraction                            -
MISSING       [5][6]
                   2.4 Simple Word Boundaries              - done
                   2.5 Simple Loose Matches                - done
                   2.6   End  of  Line                          -
MISSING       [9][10]
                   [ 1] .}
                   [ 2] .}
                   [            3]            .             {...}
                   [  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 ,  and

                        (should  also  affect  <>, $., and script
line numbers)
                        (the ,  and  do match

           [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


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

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

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

       o   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 at this level . should equal that
                   [15] need three classes, not just 72W
                   [16] see UTR#21 Case Mappings

       o   Level 3 - Locale-Sensitive Support

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

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

       Unicode Encodings    [Toc]    [Back]

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

       o   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

           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
             U+40000..U+FFFFF        F1..F3     80..BF     80..BF
            U+100000..U+10FFFF      F4         80..8F      80..BF

           Note the "A0..BF" in "U+0800..U+0FFF", the "80..9F" in
           "U+D000...U+D7FF", the "90..B"F in "U+10000..U+3FFFF",
           and the "80...8F" in "U+100000..U+10FFFF".  The "gaps"
           are caused by legal UTF-8 avoiding non-shortest encodings:
 it is technically possible to UTF-8-encode a
           single code point in different ways, but that is
           explicitly forbidden, and the shortest possible encoding
 should always be used.  So that's what Perl  does.

           Another way to look at it is via bits:

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

                               0aaaaaaa     0aaaaaaa
                       00000bbbbbaaaaaa     110bbbbb  10aaaaaa
                       ccccbbbbbbaaaaaa      1110cccc    10bbbbbb
             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.

       o   UTF-EBCDIC

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

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

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

           UTF-16 is a 2 or 4 byte encoding.  The Unicode code
           points "U+0000..U+FFFF" are stored in a single 16-bit
           unit, and the code points "U+10000..U+10FFFF" in two
           16-bit units.  The latter case is using surrogates,
           the first 16-bit unit being the high surrogate, and
           the second being the low surrogate.

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

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

           and the decoding is

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

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

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

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

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

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

       o   UTF-32, UTF-32BE, UTF-32LE

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

       o   UCS-2, UCS-4

           Encodings defined by the ISO 10646 standard.  UCS-2 is
           a 16-bit encoding.  Unlike UTF-16, UCS-2 is not extensible
 beyond "U+FFFF", because it does not use surrogates.
  UCS-4 is a 32-bit encoding, functionally identical
 to UTF-32.

       o   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    [Toc]    [Back]

       o   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 overflow
 at the receiving end of a UTF-8 connection.  Perl
           always generates the shortest length UTF-8, and with
           warnings on Perl will warn about non-shortest length
           UTF-8 along with other malformations, such as the surrogates,
 which are not real Unicode code points.

       o   Regular expressions behave slightly differently
           between byte data and character (Unicode) data.  For
           example, the "word character" character class "0
           will work differently depending on if data is eightbit
 bytes or Unicode.

           In the first case, the set of "336
           either small--the default set of alphabetic characters,
 digits, and the "_"--or, if you are using a
           locale (see perllocale), the "480
           more letters according to your language and country.

           In the second case, the "504
           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 "1032
           that LATIN SMALL LETTER ETH is a letter (unless you
           happen to speak Icelandic), but Unicode does.

           As discussed elsewhere, Perl has one foot (two
           hooves?) planted in each of two worlds: the old world
           of bytes and the new world of characters, upgrading
           from bytes to characters when necessary.  If your
           legacy code does not explicitly use Unicode, no automatic
 switch-over to characters should happen.  Characters
 shouldn't get downgraded to bytes, either.  It
           is possible to accidentally mix bytes and characters,
           however (see perluniintro), in which case "192
           ular expressions might start behaving differently.
           Review your code.  Use warnings and the "strict"

       Unicode in Perl on EBCDIC    [Toc]    [Back]

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

       Locales    [Toc]    [Back]

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

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

       o   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    [Toc]    [Back]

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

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

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

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

       o   %ENV

       o   glob (aka the <*>)

       o   open, opendir, sysopen
       o   qx (aka the backtick operator), system

       o   readdir, readlink

       Forcing Unicode in Perl (Or Unforcing Unicode in Perl)    [Toc]    [Back]

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

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

       Using Unicode in XS    [Toc]    [Back]

       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.

       o   "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 character
 with code point 0..255 within the string.  Perl's
           Unicode model is not to use UTF-8 until it is absolutely

       o   "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

       o   "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

       o   "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.

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

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

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

       o   "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.

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

       o   "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.

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

       o   "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    [Toc]    [Back]

       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    [Toc]    [Back]

       When Perl exchanges data with an extension, the extension
       should be able to understand the UTF-8 flag and act
       accordingly. If the extension doesn't know about the flag,
       it's likely that the extension will return incorrectlyflagged

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

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

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

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

       Sometimes, when the extension does not convert data but
       just stores and retrieves 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

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

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

       Speed    [Toc]    [Back]

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

       In Perl 5.8.0 the slowness was often quite spectacular; in
       Perl 5.8.1 a caching scheme was introduced which will
       hopefully make the slowness somewhat less spectacular, at
       least for some operations.  In general, operations with
       UTF-8 encoded strings are still slower. As an example, the
       Unicode  properties  (character   classes)   like   "{Nd}"
       known to be quite a bit slower (5-20 times) than their
       simpler counterparts like "" (then again, there 268 Unicode
 characters matching "Nd" compared with the 10 ASCII
       characters matching "d").

       Porting code from perl-5.6.X    [Toc]    [Back]

       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.

       o   A filehandle that should read or write UTF-8

             if ($] > 5.007) {
               binmode $fh, ":utf8";

       o   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

             if ($] > 5.007) {
               require Encode;
               $val = Enco

 Similar pages
Name OS Title
perluniintro OpenBSD Perl Unicode introduction
iso10646 Tru64 Support for the Unicode and ISO/IEC 10646 standards
UTF-8 Tru64 Support for the Unicode and ISO/IEC 10646 standards
Unicode Tru64 Support for the Unicode and ISO/IEC 10646 standards
UTF-16 Tru64 Support for the Unicode and ISO/IEC 10646 standards
UTF-32 Tru64 Support for the Unicode and ISO/IEC 10646 standards
unicode Tru64 Support for the Unicode and ISO/IEC 10646 standards
UCS-4 Tru64 Support for the Unicode and ISO/IEC 10646 standards
universal.utf8 Tru64 Support for the Unicode and ISO/IEC 10646 standards
UCS-2 Tru64 Support for the Unicode and ISO/IEC 10646 standards
Copyright © 2004-2005 DeniX Solutions SRL
newsletter delivery service