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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 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 encod-
	   ings can be converted to Perl's encoding on input or from Perl's encoding on output by
	   use of the ":encoding(...)"	layer.	See open.

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

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

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

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

       Byte and Character Semantics

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

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

       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 seman-
       tics.  For operations where this determination cannot be made without additional informa-
       tion from the user, Perl decides in favor of compatibility and chooses to use byte seman-

       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 liter-
       als and constants in the source text.

       On Windows platforms, if the "-C" command line switch is used or the ${^WIDE_SYSTEM_CALLS}
       global flag is set to 1, all system calls will use the corresponding wide-character APIs.
       This feature is available only on Windows to conform to the API standard already estab-
       lished for that platform--and there are very few non-Windows platforms that have Unicode-
       aware APIs.

       The "bytes" pragma will always, regardless of platform, force byte semantics in a particu-
       lar 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 transpar-
       ently.  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 pro-
       gram--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 con-
       catenated, the new string will be upgraded to ISO 8859-1 (Latin-1), even if the old Uni-
       code string used EBCDIC.  This translation is done without regard to the system's native
       8-bit encoding, so to change this for systems with non-Latin-1 and non-EBCDIC native
       encodings use the "encoding" pragma.  See encoding.

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

       Effects of Character Semantics

       Character semantics have the following effects:

       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 "\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}".

       o   If an appropriate encoding is specified, identifiers within the Perl script may con-
	   tain 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 "\C" pattern is provided to force a match a single byte--a
	   "char" in C, hence "\C".

       o   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.

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

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

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

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

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

	       Short	   Long

	       L	   Letter
	       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.  "L&" is a special case, which is an alias for "Ll",
	   "Lu", and "Lt".

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

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

	       Property    Meaning

	       BidiL	   Left-to-Right
	       BidiLRE	   Left-to-Right Embedding
	       BidiLRO	   Left-to-Right Override
	       BidiR	   Right-to-Left
	       BidiAL	   Right-to-Left Arabic
	       BidiRLE	   Right-to-Left Embedding
	       BidiRLO	   Right-to-Left Override
	       BidiPDF	   Pop Directional Format
	       BidiEN	   European Number
	       BidiES	   European Number Separator
	       BidiET	   European Number Terminator
	       BidiAN	   Arabic Number
	       BidiCS	   Common Number Separator
	       BidiNSM	   Non-Spacing Mark
	       BidiBN	   Boundary Neutral
	       BidiB	   Paragraph Separator
	       BidiS	   Segment Separator
	       BidiWS	   Whitespace
	       BidiON	   Other Neutrals

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


       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 punctua-
       tion, are in a category called "Common".

       For more about scripts, see the UTR #24:


       For more about blocks, see:


       Block names are given with the "In" prefix. For example, the Katakana block is referenced
       via "\p{InKatakana}".  The "In" prefix may be omitted if there is no naming conflict with
       a script or any other property, but it is recommended that "In" always be used for block
       tests to avoid confusion.

       These block names are supported:


       o   The special pattern "\X" matches any extended Unicode 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.  "\X" is equivalent
	   to "(?:\PM\pM*)".

       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 "\u" 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()", "substr()", "pos()",
	   "index()", "rindex()", "sprintf()", "write()", and "length()".  Operators that specif-
	   ically do not switch include "vec()", "pack()", and "unpack()".  Operators that really
	   don't care include "chomp()", 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 impor-
	   tantly, 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 cases:

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

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

	   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

       You can define your own character properties by defining subroutines whose names begin
       with "In" or "Is".  The subroutines must be defined in the "main" package.  The user-
       defined properties can be used in the regular expression "\p" and "\P" constructs.  Note
       that the effect is compile-time and immutable once defined.

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

       o   Two hexadecimal numbers separated by horizontal whitespace (space or tabular charac-
	   ters) denoting a range of Unicode code points to include.

       o   Something to include, prefixed by "+": a built-in character property (prefixed by
	   "utf8::"), 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::"), for 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::") for all the characters except the characters in the property; two hexadeci-
	   mal code points for a range; or a single hexadecimal code point.

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

	   sub InKana {
	       return <<END;

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

       You could also have used the existing block property names:

	   sub InKana {
	       return <<'END';

       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 classes.

	   sub InNotKana {
	       return <<'END';

       You can also define your own mappings to be used in the lc(), lcfirst(), uc(), and
       ucfirst() (or their string-inlined versions).  The principle is the same: define subrou-
       tines in the "main" package with names like "ToLower" (for lc() and lcfirst()), "ToTitle"
       (for the first character in ucfirst()), and "ToUpper" (for uc(), and the rest of the char-
       acters 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 desti-
       nation 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 tabula-
       tor characters are still needed.  For example:

	   sub ToLower {
	       return <<END;

       defines a lc() mapping that causes only "A" to be mapped to "a", all other characters will
       remain unchanged.

       (For serious hackers only)  If you want to introspect the default mappings, you can find
       the data in the directory $Config{privlib}/unicore/To/.	The mapping data is returned as
       the here-document, and the "utf8::ToSpecFoo" are special exception mappings derived from
       <$Config{privlib}>/unicore/SpecialCasing.txt.  The "Digit" and "Fold" mappings that one
       can see in the directory are not directly user-accessible, one can use either the "Uni-
       code::UCD" module, or just match case-insensitively (that's when the "Fold" mapping is

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

       Character Encodings for Input and Output

       See Encode.

       Unicode Regular Expression Support Level

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

       o   Level 1 - Basic Unicode Support

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

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

	   [a] You can mimic class subtraction using lookahead.  For example, what TR18 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 script.

	   [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 \X but at this level . should equal that
		   [15] need three classes, not just \w and \W
		   [16] see UTR#21 Case Mappings

       o   Level 3 - Locale-Sensitive Support

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

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

       Unicode Encodings

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

       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 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 that's what Perl does.

	   Another way to look at it is via bits:

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

			       0aaaaaaa     0aaaaaaa
		       00000bbbbbaaaaaa     110bbbbb  10aaaaaa
		       ccccbbbbbbaaaaaa     1110cccc  10bbbbbb	10aaaaaa
	     00000dddccccccbbbbbbaaaaaa     11110ddd  10cccccc	10bbbbbb  10aaaaaa

	   As you can see, the continuation bytes all begin with 10, and the leading bits of the
	   start byte tell how many bytes the are in the encoded character.

       o   UTF-EBCDIC

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

       o   UTF-16, UTF-16BE, UTF16-LE, Surrogates, and BOMs (Byte Order Marks)

	   The followings items are mostly for reference and general Unicode knowledge, Perl
	   doesn't 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 Uni-
	   code 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 surro-
	   gate 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 char-

	   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 guaran-
	   teed 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", rep-
	   resented in big-endian format".

       o   UTF-32, UTF-32BE, UTF32-LE

	   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

       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 malforma-
	   tions, such as the surrogates, which are not real Unicode code points.

       o   Regular expressions behave slightly differently between byte data and character (Uni-
	   code) data.	For example, the "word character" character class "\w" will work differ-
	   ently 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 perllo-
	   cale), the "\w" might contain a few more letters according to your language and coun-

	   In the second case, the "\w" set of characters is much, much larger.  Most impor-
	   tantly, even in the set of the first 256 characters, it will probably match different
	   characters: unlike most locales, which are specific to a language and country pair,
	   Unicode classifies all the characters that are letters somewhere as "\w".  For exam-
	   ple, your locale might not think 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 char-
	   acters when necessary.  If your legacy code does not explicitly use Unicode, no auto-
	   matic 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 "\w" in regular expressions might start behaving differ-
	   ently.  Review your code.  Use warnings and the "strict" pragma.

       Unicode in Perl on EBCDIC

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


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

       o   If your locale environment variables (LANGUAGE, LC_ALL, LC_CTYPE, LANG) contain the
	   strings 'UTF-8' or 'UTF8' (case-insensitive matching), the default encodings of your
	   STDIN, STDOUT, and STDERR, and of any subsequent file open, are considered to be

       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

       Using Unicode in XS

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

       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 rep-
	   resentation of the scalar is the sequence of UTF-8 encoded code points of the charac-
	   ters of a string.  The "UTF8" flag being off means that each octet in this representa-
	   tion encodes a single character with code point 0..255 within the string.  Perl's Uni-
	   code model is not to use UTF-8 until it is absolutely necessary.

       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 bytes.

       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 sequence.

       o   "utf8_length(start, end)" returns the length of the UTF-8 encoded buffer in charac-
	   ters.  "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 gen-
	   eral-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 com-
	   puting 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 distribu-

       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 demon-
       strably incorrect for locales that use characters above that range when mapped into Uni-
       code.  Perl's Unicode support will also tend to run slower.  Use of locales with Unicode
       is discouraged.

       Interaction with Extensions

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

       So if you're working with Unicode data, consult the documentation of every module you're
       using if there are any issues with Unicode data exchange. If the documentation does not
       talk about Unicode at all, suspect the worst and probably look at the source to learn how
       the module is implemented. Modules written completely in Perl shouldn't cause problems.
       Modules that directly or indirectly access code written in other programming 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 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::fil-
       ter_store_key and family. Look out for such filters in the documentation of your exten-
       sions, they can make the transition to Unicode data much easier.


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

	 % perl -e '
	 use Benchmark;
	 use strict;
	 our $l = 10000;
	 our $u = our $b = "x" x $l;
	 substr($u,0,1) = "\x{100}";
	 LENGTH_B => q{ length($b) },
	 LENGTH_U => q{ length($u) },
	 SUBSTR_B => q{ substr($b, $l/4, $l/2) },
	 SUBSTR_U => q{ substr($u, $l/4, $l/2) },
	 Benchmark: running LENGTH_B, LENGTH_U, SUBSTR_B, SUBSTR_U for at least 2 CPU seconds...
	   LENGTH_B:  2 wallclock secs ( 2.36 usr +  0.00 sys =  2.36 CPU) @ 5649983.05/s (n=13333960)
	   LENGTH_U:  2 wallclock secs ( 2.11 usr +  0.00 sys =  2.11 CPU) @ 12155.45/s (n=25648)
	   SUBSTR_B:  3 wallclock secs ( 2.16 usr +  0.00 sys =  2.16 CPU) @ 374480.09/s (n=808877)
	   SUBSTR_U:  2 wallclock secs ( 2.11 usr +  0.00 sys =  2.11 CPU) @ 6791.00/s (n=14329)

       The numbers show an incredible slowness on long UTF-8 strings.  You should carefully avoid
       using these functions in tight loops. If you want to iterate over characters, the superior
       coding technique would split the characters into an array instead of using substr, as the
       following benchmark shows:

	 % perl -e '
	 use Benchmark;
	 use strict;
	 our $l = 10000;
	 our $u = our $b = "x" x $l;
	 substr($u,0,1) = "\x{100}";
	 SPLIT_B => q{ for my $c (split //, $b){}  },
	 SPLIT_U => q{ for my $c (split //, $u){}  },
	 SUBSTR_B => q{ for my $i (0..length($b)-1){my $c = substr($b,$i,1);} },
	 SUBSTR_U => q{ for my $i (0..length($u)-1){my $c = substr($u,$i,1);} },
	 Benchmark: running SPLIT_B, SPLIT_U, SUBSTR_B, SUBSTR_U for at least 5 CPU seconds...
	    SPLIT_B:  6 wallclock secs ( 5.29 usr +  0.00 sys =  5.29 CPU) @ 56.14/s (n=297)
	    SPLIT_U:  5 wallclock secs ( 5.17 usr +  0.01 sys =  5.18 CPU) @ 55.21/s (n=286)
	   SUBSTR_B:  5 wallclock secs ( 5.34 usr +  0.00 sys =  5.34 CPU) @ 123.22/s (n=658)
	   SUBSTR_U:  7 wallclock secs ( 6.20 usr +  0.00 sys =  6.20 CPU) @  0.81/s (n=5)

       Even though the algorithm based on "substr()" is faster than "split()" for byte-encoded
       data, it pales in comparison to the speed of "split()" when used with UTF-8 data.

       Porting code from perl-5.6.X

       Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer was required to use
       the "utf8" pragma to declare that a given scope expected to deal with Unicode data and had
       to make sure that only Unicode data were reaching that scope. If you have code that is
       working with 5.6, you will need some of the following adjustments to your code. The exam-
       ples 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 true.

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

       o   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);

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

	     if ($] > 5.007) {
	       require Encode;

       o   A wrapper for fetchrow_array and fetchrow_hashref

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

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

       o   A large scalar that you know can only contain ASCII

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

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

       perluniintro, encoding, Encode, open, utf8, bytes, perlretut, "${^WIDE_SYSTEM_CALLS}" in

perl v5.8.0				    2003-02-18				   PERLUNICODE(1)

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