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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



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

           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.



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       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:

       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 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}".

       o   If an appropriate encoding is specified, identifiers



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           within the Perl script may contain Unicode
           alphanumeric characters, including ideographs.  Perl
           does not currently attempt to canonicalize variable
           names.

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

       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 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.

       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()", "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



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           strings as a bucket of bits such as "sort()", and
           operators dealing with filenames.

       o   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.

       o   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.

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

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



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           string-inlined versions).

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

       o   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




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               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:



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               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



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               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:












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               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)



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                              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



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               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



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               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



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               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:




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

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

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

       o   Something to 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.

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

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

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





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           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()).




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



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       "Unicode Regular Expressions", version 11, in May 2005.

       o   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



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           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}").

       o   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).

       o   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



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                   [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.

       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.



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       o   UTF-EBCDIC

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

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

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



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           read the bytes "0xFF 0xFE".  (And if the originating
           platform was writing in UTF-8, you will read the bytes
           "0xEF 0xBB 0xBF".)

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

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

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

       o   UCS-2, UCS-4

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

       o   UTF-7

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

   Security Implications of Unicode
       o   Malformed UTF-8

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

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



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           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:

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

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



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           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?

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

       o   %ENV

       o   glob (aka the <*>)

       o   open, opendir, sysopen

       o   qx (aka the backtick operator), system

       o   readdir, readlink

   Forcing Unicode in Perl (Or Unforcing Unicode in Perl)
       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.




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       o   "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.

       o   "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.

       o   "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.

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

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

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

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

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



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           the characters of a UTF-8 encoded buffer; "UNISKIP()"
           is useful, for example, in computing the size required
           for a UTF-8 encoded buffer.

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

       o   "utf8_hop(s, off)" will return a pointer to 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.

       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, 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



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       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



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       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



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

       o   A filehandle that should read or write UTF-8

             if ($] > 5.007) {
               binmode $fh, ":encoding(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 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
             }

       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 UTF8 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 ($] > 5.007) {
               require Encode;
               Encode::_utf8_on($val);
             }

       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



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           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 UTF8 flag:

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

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














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