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NAME ^

perlunicode - Unicode support in Perl

DESCRIPTION ^

Important Caveats

WARNING: While the implementation of Unicode support in Perl is now fairly complete it is still evolving to some extent.

In particular the way Unicode is handled on EBCDIC platforms is still rather experimental. On such a platform references to UTF-8 encoding in this document and elsewhere should be read as meaning UTF-EBCDIC as 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 re-used to mean platform's "natural" 8-bit encoding of Unicode. See perlebcdic for more discussion of the issues.

The following areas are still under development.

Input and Output Disciplines

A filehandle can be marked as containing perl's internal Unicode encoding (UTF-8 or UTF-EBCDIC) by opening it 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. There is not yet a clean way to mark the Perl source itself as being in an particular encoding.

Regular Expressions

The regular expression compiler does now attempt to produce polymorphic opcodes. That is the pattern should now adapt to the data and automatically switch to the Unicode character scheme when presented with Unicode data, or a traditional byte scheme when presented with byte data. The implementation is still new and (particularly on EBCDIC platforms) may need further work.

use utf8 still needed to enable a few features

The utf8 pragma implements the tables used for Unicode support. These tables are automatically loaded on demand, so the utf8 pragma need not normally be used.

However, as a compatibility measure, this pragma must be explicitly used to enable recognition of UTF-8 encoded literals and identifiers in the source text on ASCII based machines or recognize UTF-EBCDIC encoded literals and identifiers on EBCDIC based machines.

Byte and Character semantics

Beginning with version 5.6, Perl uses logically wide characters to represent strings internally. This internal representation of strings uses either the UTF-8 or the UTF-EBCDIC encoding.

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

However, as strictly 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 is characters, Perl now switches to character semantics. For operations where this determination cannot be made without additional information from the user, Perl decides in favor of compatibility, and chooses to use byte semantics.

This behavior preserves compatibility with earlier versions of Perl, which allowed byte semantics in Perl operations, but only as long as none of the program's inputs are 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.

If the -C command line switch is used, (or the ${^WIDE_SYSTEM_CALLS} global flag is set to 1), all system calls will use the corresponding wide character APIs. Note that this is currently only implemented on Windows since other platforms API standard on this area.

Regardless of the above, the bytes pragma can always be used to 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. It may also be used for enabling some of the more experimental Unicode support features. Note that this pragma is only required until a future version of Perl in which character semantics will become the default. This pragma may then become a no-op. See utf8.

Unless mentioned otherwise, Perl operators will use character semantics when they are dealing with Unicode data, and byte semantics otherwise. Thus, character semantics for these operations apply transparently; if the input data came from a Unicode source (for example, by adding a character encoding discipline to the filehandle whence it came, or a literal UTF-8 string constant in the program), character semantics apply; otherwise, byte semantics are in effect. To force byte semantics on Unicode data, the bytes pragma should be used.

Under character semantics, many operations that formerly operated on bytes change to operating on characters. For ASCII data this makes no difference, because UTF-8 stores ASCII in single bytes, but for any character greater than chr(127), the character may be stored in a sequence of two or more bytes, all of which have the high bit set.

For C1 controls or Latin 1 characters on an EBCDIC platform the character may be stored in a UTF-EBCDIC multi byte sequence. But by and large, the user need not worry about this, because Perl hides it from the user. A character in Perl is logically just a number ranging from 0 to 2**32 or so. Larger characters encode to longer sequences of bytes internally, but again, this is just an internal detail which is hidden at the Perl level.

Effects of character semantics

Character semantics have the following effects:

Scripts

The scripts available for \p{In...} and \P{In...}, for example \p{InCyrillic>, are as follows, for example \p{InLatin} or \P{InHan}:

   Latin
   Greek
   Cyrillic
   Armenian
   Hebrew
   Arabic
   Syriac
   Thaana
   Devanagari
   Bengali
   Gurmukhi
   Gujarati
   Oriya
   Tamil
   Telugu
   Kannada
   Malayalam
   Sinhala
   Thai
   Lao
   Tibetan
   Myanmar
   Georgian
   Hangul
   Ethiopic
   Cherokee
   CanadianAboriginal
   Ogham
   Runic
   Khmer
   Mongolian
   Hiragana
   Katakana
   Bopomofo
   Han
   Yi
   OldItalic
   Gothic
   Deseret
   Inherited

Blocks

In addition to scripts, Unicode also defines blocks of characters. The difference between scripts and blocks is that the former concept is closer to natural languages, while the latter concept is more an artificial grouping based on groups of 256 Unicode characters. For example, the Latin script contains letters from many blocks, but it does not contain all the characters from those blocks, it does not for example contain digits.

For more about scripts see the UTR #24: http://www.unicode.org/unicode/reports/tr24/ For more about blocks see http://www.unicode.org/Public/UNIDATA/Blocks.txt

Because there are overlaps in naming (there are, for example, both a script called Katakana and a block called Katakana, the block version has Block appended to its name, \p{InKatakanaBlock}.

Notice that this definition was introduced in Perl 5.8.0: in Perl 5.6.0 only the blocks were used; in Perl 5.8.0 scripts became the preferential character class definition; this meant that the definitions of some character classes changed (the ones in the below list that have the Block appended).

   BasicLatin
   Latin1Supplement
   LatinExtendedA
   LatinExtendedB
   IPAExtensions
   SpacingModifierLetters
   CombiningDiacriticalMarks
   GreekBlock
   CyrillicBlock
   ArmenianBlock
   HebrewBlock
   ArabicBlock
   SyriacBlock
   ThaanaBlock
   DevanagariBlock
   BengaliBlock
   GurmukhiBlock
   GujaratiBlock
   OriyaBlock
   TamilBlock
   TeluguBlock
   KannadaBlock
   MalayalamBlock
   SinhalaBlock
   ThaiBlock
   LaoBlock
   TibetanBlock
   MyanmarBlock
   GeorgianBlock
   HangulJamo
   EthiopicBlock
   CherokeeBlock
   UnifiedCanadianAboriginalSyllabics
   OghamBlock
   RunicBlock
   KhmerBlock
   MongolianBlock
   LatinExtendedAdditional
   GreekExtended
   GeneralPunctuation
   SuperscriptsandSubscripts
   CurrencySymbols
   CombiningMarksforSymbols
   LetterlikeSymbols
   NumberForms
   Arrows
   MathematicalOperators
   MiscellaneousTechnical
   ControlPictures
   OpticalCharacterRecognition
   EnclosedAlphanumerics
   BoxDrawing
   BlockElements
   GeometricShapes
   MiscellaneousSymbols
   Dingbats
   BraillePatterns
   CJKRadicalsSupplement
   KangxiRadicals
   IdeographicDescriptionCharacters
   CJKSymbolsandPunctuation
   HiraganaBlock
   KatakanaBlock
   BopomofoBlock
   HangulCompatibilityJamo
   Kanbun
   BopomofoExtended
   EnclosedCJKLettersandMonths
   CJKCompatibility
   CJKUnifiedIdeographsExtensionA
   CJKUnifiedIdeographs
   YiSyllables
   YiRadicals
   HangulSyllables
   HighSurrogates
   HighPrivateUseSurrogates
   LowSurrogates
   PrivateUse
   CJKCompatibilityIdeographs
   AlphabeticPresentationForms
   ArabicPresentationFormsA
   CombiningHalfMarks
   CJKCompatibilityForms
   SmallFormVariants
   ArabicPresentationFormsB
   Specials
   HalfwidthandFullwidthForms
   OldItalicBlock
   GothicBlock
   DeseretBlock
   ByzantineMusicalSymbols
   MusicalSymbols
   MathematicalAlphanumericSymbols
   CJKUnifiedIdeographsExtensionB
   CJKCompatibilityIdeographsSupplement
   Tags

Character encodings for input and output

See Encode.

CAVEATS ^

As of yet, there is no method for automatically coercing input and output to some encoding other than UTF-8 or UTF-EBCDIC. This is planned in the near future, however.

Whether an arbitrary piece of data will be treated as "characters" or "bytes" by internal operations cannot be divined at the current time.

Use of locales with utf8 may lead to odd results. Currently there is some attempt to apply 8-bit locale info to characters in the range 0..255, but this is demonstrably incorrect for locales that use characters above that range (when mapped into Unicode). It will also tend to run slower. Avoidance of locales is strongly encouraged.

SEE ALSO ^

bytes, utf8, perlretut, "${^WIDE_SYSTEM_CALLS}" in perlvar

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