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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 imple
       ment the Unicode standard or the accompanying technical reports from
       cover to cover, Perl does support many Unicode features.

       Input and Output Layers
	   Perl knows when a filehandle uses Perls 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 Perls
	   encoding on input or from Perls 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

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

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

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

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

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

	       use encoding utf8;

	   See "Byte and Character Semantics" for more details.

       Byte and Character Semantics

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

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

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

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

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

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

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

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

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

       Effects of Character Semantics

       Character semantics have the following effects:

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

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

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

	   Additionally, if you

	      use charnames :full;

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

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

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

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

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

	  Named Unicode properties, scripts, and block ranges may be used
	   like character classes via the "\p{}" "matches property" construct
	   and the  "\P{}" negation, "doesnt match property".

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

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

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

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

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

	       Short	   Long

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

	       M	   Mark
	       Mn	   NonspacingMark
	       Mc	   SpacingMark
	       Me	   EnclosingMark

	       N	   Number
	       Nd	   DecimalNumber
	       Nl	   LetterNumber
	       No	   OtherNumber

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

	       S	   Symbol
	       Sm	   MathSymbol
	       Sc	   CurrencySymbol
	       Sk	   ModifierSymbol
	       So	   OtherSymbol

	       Z	   Separator
	       Zs	   SpaceSeparator
	       Zl	   LineSeparator
	       Zp	   ParagraphSeparator

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

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

	   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

	   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 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 \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 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

       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 omit
       ted if there is no naming conflict with a script or any other property,
       but it is recommended that "In" always be used for block tests to avoid

       These block names are supported:


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

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

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

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

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

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

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

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

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

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

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

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

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

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

       User-Defined Character Properties

       You can define your own character properties by defining subroutines
       whose names begin with "In" or "Is".  The subroutines can be defined in
       any package.  The user-defined properties can be used in the regular
       expression "\p" and "\P" constructs; if you are using a user-defined
       property from a package other than the one you are in, you must specify
       its package in the "\p" or "\P" construct.

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

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

       Note that the effect is compile-time and immutable once defined.

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

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

	  Something to include, prefixed by "+": a built-in character prop
	   erty (prefixed by "utf8::") or a user-defined character property,
	   to represent all the characters in that property; two hexadecimal
	   code points for a range; or a single hexadecimal code point.

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

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

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

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

	   sub InKana {
	       return </unicore/SpecialCasing.txt.  The "Digit" and "Fold" map
       pings that one can see in the directory are not directly user-accessi
       ble, one can use either the "Unicode::UCD" module, or just match case-
       insensitively (thats when the "Fold" mapping is used).

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

       Character Encodings for Input and Output

       See Encode.

       Unicode Regular Expression Support Level

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

	  Level 1 - Basic Unicode Support

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

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

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


	   in Perl can be written as:


	   But in this particular example, you probably really want


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

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

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

	  Level 3 - Locale-Sensitive Support

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

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

       Unicode Encodings

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


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

	   The following table is from Unicode 3.2.

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

	      U+0000..U+007F	   00..7F
	      U+0080..U+07FF	   C2..DF    80..BF
	      U+0800..U+0FFF	   E0	     A0..BF    80..BF
	      U+1000..U+CFFF	   E1..EC    80..BF    80..BF
	      U+D000..U+D7FF	   ED	     80..9F    80..BF
	      U+D800..U+DFFF	   ******* ill-formed *******
	      U+E000..U+FFFF	   EE..EF    80..BF    80..BF
	     U+10000..U+3FFFF	   F0	     90..BF    80..BF	 80..BF
	     U+40000..U+FFFFF	   F1..F3    80..BF    80..BF	 80..BF
	    U+100000..U+10FFFF	   F4	     80..8F    80..BF	 80..BF

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


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

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

	   The followings items are mostly for reference and general Unicode
	   knowledge, Perl doesnt use these constructs internally.

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

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

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

	   and the decoding is

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

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

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

	   This introduces another problem: what if you just know that your
	   data is UTF-16, but you dont know which endianness?	Byte Order
	   Marks, or BOMs, are a solution to this.  A special character has
	   been reserved in Unicode to function as a byte order marker: the
	   character with the code point "U+FEFF" is the BOM.

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

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

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

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

	  UCS-2, UCS-4

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


	   A seven-bit safe (non-eight-bit) encoding, which is useful if the
	   transport or storage is not eight-bit safe.	Defined by RFC 2152.

       Security Implications of Unicode

	  Malformed UTF-8

	   Unfortunately, the specification of UTF-8 leaves some room for
	   interpretation of how many bytes of encoded output one should gen
	   erate from one input Unicode character.  Strictly speaking, the
	   shortest possible sequence of UTF-8 bytes should be generated,
	   because otherwise there is potential for an input buffer overflow
	   at the receiving end of a UTF-8 connection.	Perl always generates
	   the shortest length UTF-8, and with warnings on Perl will warn
	   about non-shortest length UTF-8 along with other malformations,
	   such as the surrogates, which are not real Unicode code points.

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

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

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

	   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 shouldnt get
	   downgraded to bytes, either.  It is possible to accidentally mix
	   bytes and characters, however (see perluniintro), in which case
	   "\w" in regular expressions might start behaving differently.
	   Review your code.  Use warnings and the "strict" pragma.

       Unicode in Perl on EBCDIC

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


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

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

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

       When Unicode Does Not Happen

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

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

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

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


	  glob (aka the <*>)

	  open, opendir, sysopen

	  qx (aka the backtick operator), system

	  readdir, readlink

       Forcing Unicode in Perl (Or Unforcing Unicode in Perl)

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

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

       Using Unicode in XS

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       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.  Perls Uni
       code support will also tend to run slower.  Use of locales with Unicode
       is discouraged.

       Interaction with Extensions

       When Perl exchanges data with an extension, the extension should be
       able to understand the UTF-8 flag and act accordingly. If the extension
       doesnt know about the flag, its likely that the extension will return
       incorrectly-flagged data.

       So if youre working with Unicode data, consult the documentation of
       every module youre using if there are any issues with Unicode data
       exchange. If the documentation does not talk about Unicode at all, sus
       pect the worst and probably look at the source to learn how the module
       is implemented. Modules written completely in Perl shouldnt 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, lets say the popular Foo::Bar::escape_html
       function doesnt deal with Unicode data yet. The wrapper function would
       convert the argument to raw UTF-8 and convert the result back to Perls
       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. Lets say the popular "Foo::Bar"
       extension, written in C, provides a "param" method that lets you store
       and retrieve data according to these prototypes:

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

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

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

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


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

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

       Porting code from perl-5.6.X

       Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
       was required to use the "utf8" pragma to declare that a given scope
       expected to deal with Unicode data and had to make sure that only Uni
       code data were reaching that scope. If you have code that is working
       with 5.6, you will need some of the following adjustments to your code.
       The examples are written such that the code will continue to work under
       5.6, so you should be safe to try them out.

	  A filehandle that should read or write UTF-8

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

	  A scalar that is going to be passed to some extension

	   Be it Compress::Zlib, Apache::Request or any extension that has no
	   mention of Unicode in the manpage, you need to make sure that the
	   UTF-8 flag is stripped off. Note that at the time of this writing
	   (October 2002) the mentioned modules are not UTF-8-aware. Please
	   check the documentation to verify if this is still true.

	     if ($] > 5.007) {
	       require Encode;
	       $val = Encode::encode_utf8($val); # make octets

	  A scalar we got back from an extension

	   If you believe the scalar comes back as UTF-8, you will most likely
	   want the UTF-8 flag restored:

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

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

	     if ($] > 5.007) {
	       require Encode;

	  A wrapper for fetchrow_array and fetchrow_hashref

	   When the database contains only UTF-8, a wrapper function or method
	   is a convenient way to replace all your fetchrow_array and
	   fetchrow_hashref calls. A wrapper function will also make it easier
	   to adapt to future enhancements in your database driver. Note that
	   at the time of this writing (October 2002), the DBI has no stan
	   dardized way to deal with UTF-8 data. Please check the documenta
	   tion to verify if that is still true.

	     sub fetchrow {
	       my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref}
	       if ($] < 5.007) {
		 return $sth->$what;
	       } else {
		 require Encode;
		 if (wantarray) {
		   my @arr = $sth->$what;
		   for (@arr) {
		     defined && /[^\000-\177]/ && Encode::_utf8_on($_);
		   return @arr;
		 } else {
		   my $ret = $sth->$what;
		   if (ref $ret) {
		     for my $k (keys %$ret) {
		       defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret->{$k};
		     return $ret;
		   } else {
		     defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
		     return $ret;

	  A large scalar that you know can only contain ASCII

	   Scalars that contain only ASCII and are marked as UTF-8 are some
	   times a drag to your program. If you recognize such a situation,
	   just remove the UTF-8 flag:

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

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

perl v5.8.8			  2008-04-25			PERLUNICODE(1)

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