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

NAME
       perlunicode - Unicode support in Perl

DESCRIPTION
   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.

       People who want to learn to use Unicode in Perl, should probably read
       the Perl Unicode tutorial, perlunitut, before reading this reference
       document.

       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 in UTF-8, use "use utf8;".

       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 data that is
	   internally encoded in UTF-8 -- 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.

       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 Latin-1.

	   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
       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.

       Under byte semantics, when "use locale" is in effect, Perl uses the
       semantics associated with the current locale.  Absent a "use locale",
       Perl currently uses US-ASCII (or Basic Latin in Unicode terminology)
       byte semantics, meaning that characters whose ordinal numbers are in
       the range 128 - 255 are undefined except for their ordinal numbers.
       This means that none have case (upper and lower), nor are any a member
       of character classes, like "[:alpha:]" or "\w".	(But all do belong to
       the "\W" class or the Perl regular expression extension "[:^alpha:]".)

       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 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 have character
       semantics.  This can cause surprises: See "BUGS", below

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

   Effects of Character Semantics
       Character semantics have the following effects:

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

	   If you use a Unicode editor to edit your program, Unicode
	   characters may occur directly within the literal strings in UTF-8
	   encoding, or UTF-16.	 (The former requires a BOM or "use utf8", the
	   latter requires a BOM.)

	   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 works for all characters,
	   but for characters under 0x100, note that Perl may use an 8 bit
	   encoding internally, for optimization and/or backward
	   compatibility.

	   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
	   variable names.

       ·   Regular expressions match characters instead of bytes.  "." matches
	   a character instead of a byte.

       ·   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.

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

	   See "Unicode Character Properties" for more details.

	   You can define your own character properties and use them in the
	   regular expression with the "\p{}" or "\P{}" construct.

	   See "User-Defined Character Properties" for more details.

       ·   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*)".

       ·   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()".  An operator that
	   specifically does not switch is "vec()".  Operators that really
	   don't care include operators that treat strings as a bucket of bits
	   such as "sort()", and operators dealing with filenames.

       ·   The "pack()"/"unpack()" letter "C" does not change, since it is
	   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. There is also a "W" specifier that is
	   the equivalent of "chr"/"ord" and properly handles character values
	   even if they are above 255.

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

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

       ·   lc(), uc(), lcfirst(), and ucfirst() work for the 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
	   details.

	   But you can also define your own mappings to be used in the lc(),
	   lcfirst(), uc(), and ucfirst() (or their string-inlined versions).

	   See "User-Defined Case Mappings" for more details.

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

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

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

       The official Unicode script and block names have spaces and dashes as
       separators, but for convenience you can use dashes, spaces, or
       underbars, and case is unimportant. It is recommended, however, that
       for 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 "\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
       5.0.0 in July 2006.

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

       Bidirectional Character Types
	   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.

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

	       Arabic
	       Armenian
	       Balinese
	       Bengali
	       Bopomofo
	       Braille
	       Buginese
	       Buhid
	       CanadianAboriginal
	       Cherokee
	       Coptic
	       Cuneiform
	       Cypriot
	       Cyrillic
	       Deseret
	       Devanagari
	       Ethiopic
	       Georgian
	       Glagolitic
	       Gothic
	       Greek
	       Gujarati
	       Gurmukhi
	       Han
	       Hangul
	       Hanunoo
	       Hebrew
	       Hiragana
	       Inherited
	       Kannada
	       Katakana
	       Kharoshthi
	       Khmer
	       Lao
	       Latin
	       Limbu
	       LinearB
	       Malayalam
	       Mongolian
	       Myanmar
	       NewTaiLue
	       Nko
	       Ogham
	       OldItalic
	       OldPersian
	       Oriya
	       Osmanya
	       PhagsPa
	       Phoenician
	       Runic
	       Shavian
	       Sinhala
	       SylotiNagri
	       Syriac
	       Tagalog
	       Tagbanwa
	       TaiLe
	       Tamil
	       Telugu
	       Thaana
	       Thai
	       Tibetan
	       Tifinagh
	       Ugaritic
	       Yi

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

	       ASCIIHexDigit
	       BidiControl
	       Dash
	       Deprecated
	       Diacritic
	       Extender
	       HexDigit
	       Hyphen
	       Ideographic
	       IDSBinaryOperator
	       IDSTrinaryOperator
	       JoinControl
	       LogicalOrderException
	       NoncharacterCodePoint
	       OtherAlphabetic
	       OtherDefaultIgnorableCodePoint
	       OtherGraphemeExtend
	       OtherIDStart
	       OtherIDContinue
	       OtherLowercase
	       OtherMath
	       OtherUppercase
	       PatternSyntax
	       PatternWhiteSpace
	       QuotationMark
	       Radical
	       SoftDotted
	       STerm
	       TerminalPunctuation
	       UnifiedIdeograph
	       VariationSelector
	       WhiteSpace

	   and there are further derived properties:

	       Alphabetic  =  Lu + Ll + Lt + Lm + Lo + Nl + OtherAlphabetic
	       Lowercase   =  Ll + OtherLowercase
	       Uppercase   =  Lu + OtherUppercase
	       Math	   =  Sm + OtherMath

	       IDStart	   =  Lu + Ll + Lt + Lm + Lo + Nl + OtherIDStart
	       IDContinue  =  IDStart + Mn + Mc + Nd + Pc + OtherIDContinue

	       DefaultIgnorableCodePoint
			   =  OtherDefaultIgnorableCodePoint
			      + Cf + Cc + Cs + Noncharacters + VariationSelector
			      - WhiteSpace - FFF9..FFFB (Annotation Characters)

	       Any	   =  Any code points (i.e. U+0000 to U+10FFFF)
	       Assigned	   =  Any non-Cn code points (i.e. synonym for \P{Cn})
	       Unassigned  =  Synonym for \p{Cn}
	       ASCII	   =  ASCII (i.e. U+0000 to U+007F)

	       Common	   =  Any character (or unassigned code point)
			      not explicitly assigned to a script

       Use of "Is" Prefix
	   For backward compatibility (with Perl 5.6), all properties
	   mentioned so far may have "Is" prepended to their name, so
	   "\P{IsLu}", for example, is equal to "\P{Lu}".

       Blocks
	   In addition to scripts, Unicode also defines blocks of characters.
	   The difference between scripts and blocks is that the concept of
	   scripts is closer to natural 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 UAX#24 "Script Names":

	      http://www.unicode.org/reports/tr24/

	   For more about blocks, see:

	      http://www.unicode.org/Public/UNIDATA/Blocks.txt

	   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:

	       InAegeanNumbers
	       InAlphabeticPresentationForms
	       InAncientGreekMusicalNotation
	       InAncientGreekNumbers
	       InArabic
	       InArabicPresentationFormsA
	       InArabicPresentationFormsB
	       InArabicSupplement
	       InArmenian
	       InArrows
	       InBalinese
	       InBasicLatin
	       InBengali
	       InBlockElements
	       InBopomofo
	       InBopomofoExtended
	       InBoxDrawing
	       InBraillePatterns
	       InBuginese
	       InBuhid
	       InByzantineMusicalSymbols
	       InCJKCompatibility
	       InCJKCompatibilityForms
	       InCJKCompatibilityIdeographs
	       InCJKCompatibilityIdeographsSupplement
	       InCJKRadicalsSupplement
	       InCJKStrokes
	       InCJKSymbolsAndPunctuation
	       InCJKUnifiedIdeographs
	       InCJKUnifiedIdeographsExtensionA
	       InCJKUnifiedIdeographsExtensionB
	       InCherokee
	       InCombiningDiacriticalMarks
	       InCombiningDiacriticalMarksSupplement
	       InCombiningDiacriticalMarksforSymbols
	       InCombiningHalfMarks
	       InControlPictures
	       InCoptic
	       InCountingRodNumerals
	       InCuneiform
	       InCuneiformNumbersAndPunctuation
	       InCurrencySymbols
	       InCypriotSyllabary
	       InCyrillic
	       InCyrillicSupplement
	       InDeseret
	       InDevanagari
	       InDingbats
	       InEnclosedAlphanumerics
	       InEnclosedCJKLettersAndMonths
	       InEthiopic
	       InEthiopicExtended
	       InEthiopicSupplement
	       InGeneralPunctuation
	       InGeometricShapes
	       InGeorgian
	       InGeorgianSupplement
	       InGlagolitic
	       InGothic
	       InGreekExtended
	       InGreekAndCoptic
	       InGujarati
	       InGurmukhi
	       InHalfwidthAndFullwidthForms
	       InHangulCompatibilityJamo
	       InHangulJamo
	       InHangulSyllables
	       InHanunoo
	       InHebrew
	       InHighPrivateUseSurrogates
	       InHighSurrogates
	       InHiragana
	       InIPAExtensions
	       InIdeographicDescriptionCharacters
	       InKanbun
	       InKangxiRadicals
	       InKannada
	       InKatakana
	       InKatakanaPhoneticExtensions
	       InKharoshthi
	       InKhmer
	       InKhmerSymbols
	       InLao
	       InLatin1Supplement
	       InLatinExtendedA
	       InLatinExtendedAdditional
	       InLatinExtendedB
	       InLatinExtendedC
	       InLatinExtendedD
	       InLetterlikeSymbols
	       InLimbu
	       InLinearBIdeograms
	       InLinearBSyllabary
	       InLowSurrogates
	       InMalayalam
	       InMathematicalAlphanumericSymbols
	       InMathematicalOperators
	       InMiscellaneousMathematicalSymbolsA
	       InMiscellaneousMathematicalSymbolsB
	       InMiscellaneousSymbols
	       InMiscellaneousSymbolsAndArrows
	       InMiscellaneousTechnical
	       InModifierToneLetters
	       InMongolian
	       InMusicalSymbols
	       InMyanmar
	       InNKo
	       InNewTaiLue
	       InNumberForms
	       InOgham
	       InOldItalic
	       InOldPersian
	       InOpticalCharacterRecognition
	       InOriya
	       InOsmanya
	       InPhagspa
	       InPhoenician
	       InPhoneticExtensions
	       InPhoneticExtensionsSupplement
	       InPrivateUseArea
	       InRunic
	       InShavian
	       InSinhala
	       InSmallFormVariants
	       InSpacingModifierLetters
	       InSpecials
	       InSuperscriptsAndSubscripts
	       InSupplementalArrowsA
	       InSupplementalArrowsB
	       InSupplementalMathematicalOperators
	       InSupplementalPunctuation
	       InSupplementaryPrivateUseAreaA
	       InSupplementaryPrivateUseAreaB
	       InSylotiNagri
	       InSyriac
	       InTagalog
	       InTagbanwa
	       InTags
	       InTaiLe
	       InTaiXuanJingSymbols
	       InTamil
	       InTelugu
	       InThaana
	       InThai
	       InTibetan
	       InTifinagh
	       InUgaritic
	       InUnifiedCanadianAboriginalSyllabics
	       InVariationSelectors
	       InVariationSelectorsSupplement
	       InVerticalForms
	       InYiRadicals
	       InYiSyllables
	       InYijingHexagramSymbols

   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:

       ·   A single hexadecimal number denoting a Unicode code point to
	   include.

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

       ·   Something to include, prefixed by "+": a built-in 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 exclude, prefixed by "-": 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 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
	   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 define

	   sub InKana {
	       return <<END;
	   3040\t309F
	   30A0\t30FF
	   END
	   }

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

       You could also have used the existing block property names:

	   sub InKana {
	       return <<'END';
	   +utf8::InHiragana
	   +utf8::InKatakana
	   END
	   }

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

	   sub InKana {
	       return <<'END';
	   +utf8::InHiragana
	   +utf8::InKatakana
	   -utf8::IsCn
	   END
	   }

       The negation is useful for defining (surprise!) negated classes.

	   sub InNotKana {
	       return <<'END';
	   !utf8::InHiragana
	   -utf8::InKatakana
	   +utf8::IsCn
	   END
	   }

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

	   sub InFooAndBar {
	       return <<'END';
	   +main::Foo
	   &main::Bar
	   END
	   }

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

   User-Defined Case Mappings
       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 similar to that of user-defined character properties: to
       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;
	   0061\t0063\t0041
	   END
	   }

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

       If there is no source range to speak of, that is, the 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;
	   0041\t\t0061
	   END
	   }

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

       (For serious hackers only)  If you want to introspect the default
       mappings, you can find the data in the directory
       $Config{privlib}/unicore/To/.  The mapping data is returned as the
       here-document, and the "utf8::ToSpecFoo" are special exception mappings
       derived from <$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 case 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 Standard #18,
       "Unicode Regular Expressions", version 11, in May 2005.

       ·   Level 1 - Basic Unicode Support

		   RL1.1   Hex Notation			       - done	       [1]
		   RL1.2   Properties			       - done	       [2][3]
		   RL1.2a  Compatibility Properties	       - done	       [4]
		   RL1.3   Subtraction and Intersection	       - MISSING       [5]
		   RL1.4   Simple Word Boundaries	       - done	       [6]
		   RL1.5   Simple Loose Matches		       - done	       [7]
		   RL1.6   Line Boundaries		       - MISSING       [8]
		   RL1.7   Supplementary Code Points	       - done	       [9]

		   [1]	\x{...}
		   [2]	\p{...} \P{...}
		   [3]	supports not only minimal list (general category, scripts,
			Alphabetic, Lowercase, Uppercase, WhiteSpace,
			NoncharacterCodePoint, DefaultIgnorableCodePoint, Any,
			ASCII, Assigned), but also bidirectional types, blocks, etc.
			(see "Unicode Character Properties")
		   [4]	\d \D \s \S \w \W \X [:prop:] [:^prop:]
		   [5]	can use regular expression look-ahead [a] or
			user-defined character properties [b] to emulate set operations
		   [6]	\b \B
		   [7]	note that Perl does Full case-folding in matching, not Simple:
			for example U+1F88 is equivalent to U+1F00 U+03B9,
			not with 1F80.	This difference matters mainly 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.
		   [8]	should do ^ and $ also on U+000B (\v in C), FF (\f), CR (\r),
			CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS (U+2029);
			should also affect <>, $., and script line numbers;
			should not split lines within CRLF [c] (i.e. there is no empty
			line between \r and \n)
		   [9]	UTF-8/UTF-EBDDIC used in perl allows not only U+10000 to U+10FFFF
			but also beyond U+10FFFF [d]

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

	       [{Greek}-[{UNASSIGNED}]]

	   in Perl can be written as:

	       (?!\p{Unassigned})\p{InGreekAndCoptic}
	       (?=\p{Assigned})\p{InGreekAndCoptic}

	   But in this particular example, you probably really want

	       \p{GreekAndCoptic}

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

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

	   [b] '+' for union, '-' for removal (set-difference), '&' for
	   intersection (see "User-Defined Character Properties")

	   [c] Try the ":crlf" layer (see PerlIO).

	   [d] Avoid "use warning 'utf8';" (or say "no warning 'utf8';") to
	   allow U+FFFF ("\x{FFFF}").

       ·   Level 2 - Extended Unicode Support

		   RL2.1   Canonical Equivalents	   - MISSING	   [10][11]
		   RL2.2   Default Grapheme Clusters	   - MISSING	   [12][13]
		   RL2.3   Default Word Boundaries	   - MISSING	   [14]
		   RL2.4   Default Loose Matches	   - MISSING	   [15]
		   RL2.5   Name Properties		   - MISSING	   [16]
		   RL2.6   Wildcard Properties		   - MISSING

		   [10] see UAX#15 "Unicode Normalization Forms"
		   [11] have Unicode::Normalize but not integrated to regexes
		   [12] have \X but at this level . should equal that
		   [13] UAX#29 "Text Boundaries" considers CRLF and Hangul syllable
			clusters as a single grapheme cluster.
		   [14] see UAX#29, Word Boundaries
		   [15] see UAX#21 "Case Mappings"
		   [16] have \N{...} but neither compute names of CJK Ideographs
			and Hangul Syllables nor use a loose match [e]

	   [e] "\N{...}" allows namespaces (see charnames).

       ·   Level 3 - Tailored Support

		   RL3.1   Tailored Punctuation		   - MISSING
		   RL3.2   Tailored Grapheme Clusters	   - MISSING	   [17][18]
		   RL3.3   Tailored Word Boundaries	   - MISSING
		   RL3.4   Tailored Loose Matches	   - MISSING
		   RL3.5   Tailored Ranges		   - MISSING
		   RL3.6   Context Matching		   - MISSING	   [19]
		   RL3.7   Incremental Matches		   - MISSING
		 ( RL3.8   Unicode Set Sharing )
		   RL3.9   Possible Match Sets		   - MISSING
		   RL3.10  Folded Matching		   - MISSING	   [20]
		   RL3.11  Submatchers			   - MISSING

		   [17] see UAX#10 "Unicode Collation Algorithms"
		   [18] have Unicode::Collate but not integrated to regexes
		   [19] have (?<=x) and (?=x), but look-aheads or look-behinds should see
			outside of the target substring
		   [20] need insensitive matching for linguistic features other than case;
			for example, hiragana to katakana, wide and narrow, simplified Han
			to traditional Han (see UTR#30 "Character Foldings")

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

       ·   UTF-8

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

	   The following table is from Unicode 3.2.

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

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

	   Note the "A0..BF" in "U+0800..U+0FFF", the "80..9F" in
	   "U+D000...U+D7FF", the "90..B"F in "U+10000..U+3FFFF", and the
	   "80...8F" in "U+100000..U+10FFFF".  The "gaps" are caused by legal
	   UTF-8 avoiding non-shortest 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.

       ·   UTF-EBCDIC

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

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

	   The followings items are mostly for reference and 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 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".

       ·   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.

       ·   UTF-7

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

   Security Implications of Unicode
       ·   Malformed UTF-8

	   Unfortunately, the specification of UTF-8 leaves some room for
	   interpretation of how many bytes of encoded output one should
	   generate from one input Unicode character.  Strictly speaking, the
	   shortest possible sequence of UTF-8 bytes should be generated,
	   because otherwise there is potential for an input buffer 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
	   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 example, 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 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
	   "\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 platform's "natural"
       8-bit encoding of Unicode. See perlebcdic for more discussion of the
       issues.

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

       ·   You can enable automatic UTF-8-ification of your 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.

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

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

       The following are such interfaces.  For all of these 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

       ·   %ENV

       ·   glob (aka the <*>)

       ·   open, opendir, sysopen

       ·   qx (aka the backtick operator), system

       ·   readdir, readlink

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

       Note that utf8::downgrade() can fail if the string contains characters
       that don't fit into a byte.

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

       ·   "DO_UTF8(sv)" returns true if the "UTF8" flag is on and the bytes
	   pragma is not in effect.  "SvUTF8(sv)" returns true if 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 necessary.

       ·   "uvchr_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.  It works appropriately on
	   EBCDIC machines.

       ·   "utf8_to_uvchr(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.  It works appropriately on
	   EBCDIC machines.

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

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

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

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

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

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

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

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

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

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

BUGS
   Interaction with Locales
       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.

   Problems with characters whose ordinal numbers are in the range 128 - 255
       with no Locale specified
       Without a locale specified, unlike all other characters or code points,
       these characters have very different semantics in byte semantics versus
       character semantics.  In character semantics they are interpreted as
       Unicode code points, which means they are viewed as Latin-1
       (ISO-8859-1).  In byte semantics, they are considered to be unassigned
       characters, meaning that the only semantics they have is their ordinal
       numbers, and that they are not members of various character classes.
       None are considered to match "\w" for example, but all match "\W".
       Besides these class matches, the known operations that this affects are
       those that change the case, regular expression matching while ignoring
       case, and quotemeta().  This can lead to unexpected results in which a
       string's semantics suddenly change if a code point above 255 is
       appended to or removed from it, which changes the string's semantics
       from byte to character or vice versa.  This behavior is scheduled to
       change in version 5.12, but in the meantime, a workaround is to always
       call utf8::upgrade($string), or to use the standard modules Encode or
       charnames.

   Interaction with Extensions
       When Perl exchanges data with an extension, the extension should be
       able to understand the UTF8 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 $what;
	     Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
	   }

       Sometimes, when the extension does not convert data but just stores and
       retrieves them, you will be in a position to use the otherwise
       dangerous Encode::_utf8_on() 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::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
       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").

   Possible problems on EBCDIC platforms
       In earlier versions, when byte and character data were concatenated,
       the new string was sometimes created by decoding the byte strings as
       ISO 8859-1 (Latin-1), even if the old Unicode string used EBCDIC.

       If you find any of these, please report them as bugs.

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

       ·   A filehandle that should read or write UTF-8

	     if ($] > 5.007) {
	       binmode $fh, ":encoding(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
	   UTF8 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 UTF8 flag restored:

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

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

	     if ($] > 5.007) {
	       require Encode;
	       Encode::_utf8_on($val);
	     }

       ·   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;
		   }
		 }
	       }
	     }

       ·   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 UTF8 flag:

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

SEE ALSO
       perlunitut, perluniintro, Encode, open, utf8, bytes, perlretut,
       "${^UNICODE}" in perlvar

perl v5.10.1			  2009-05-14			PERLUNICODE(1)
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