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CONTENTS

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.

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 using a particular encoding, see encoding.

Regular Expressions

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

use utf8 still needed to enable UTF-8/UTF-EBCDIC in scripts

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

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

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.

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

use encoding 'utf8';

See "Byte and Character Semantics" for more details.

Byte and Character Semantics

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

In future, Perl-level operations will be expected to work with 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.

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 be created by decoding the byte strings as ISO 8859-1 (Latin-1), even if the old Unicode string used EBCDIC. This translation is done without regard to the system's native 8-bit encoding. To change this for systems with non-Latin-1 and non-EBCDIC native encodings, use the encoding pragma. See encoding.

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

Effects of Character Semantics

Character semantics have the following effects:

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

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

ASCIIHexDigit
BidiControl
Dash
Deprecated
Diacritic
Extender
GraphemeLink
HexDigit
Hyphen
Ideographic
IDSBinaryOperator
IDSTrinaryOperator
JoinControl
LogicalOrderException
NoncharacterCodePoint
OtherAlphabetic
OtherDefaultIgnorableCodePoint
OtherGraphemeExtend
OtherLowercase
OtherMath
OtherUppercase
QuotationMark
Radical
SoftDotted
TerminalPunctuation
UnifiedIdeograph
WhiteSpace

and there are further derived properties:

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

ID_Start        Lu + Ll + Lt + Lm + Lo + Nl
ID_Continue     ID_Start + Mn + Mc + Nd + Pc

Any             Any character
Assigned        Any non-Cn character (i.e. synonym for \P{Cn})
Unassigned      Synonym for \p{Cn}
Common          Any character (or unassigned code point)
                not explicitly assigned to a script

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

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 UTR #24:

http://www.unicode.org/unicode/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:

InAlphabeticPresentationForms
InArabic
InArabicPresentationFormsA
InArabicPresentationFormsB
InArmenian
InArrows
InBasicLatin
InBengali
InBlockElements
InBopomofo
InBopomofoExtended
InBoxDrawing
InBraillePatterns
InBuhid
InByzantineMusicalSymbols
InCJKCompatibility
InCJKCompatibilityForms
InCJKCompatibilityIdeographs
InCJKCompatibilityIdeographsSupplement
InCJKRadicalsSupplement
InCJKSymbolsAndPunctuation
InCJKUnifiedIdeographs
InCJKUnifiedIdeographsExtensionA
InCJKUnifiedIdeographsExtensionB
InCherokee
InCombiningDiacriticalMarks
InCombiningDiacriticalMarksforSymbols
InCombiningHalfMarks
InControlPictures
InCurrencySymbols
InCyrillic
InCyrillicSupplementary
InDeseret
InDevanagari
InDingbats
InEnclosedAlphanumerics
InEnclosedCJKLettersAndMonths
InEthiopic
InGeneralPunctuation
InGeometricShapes
InGeorgian
InGothic
InGreekExtended
InGreekAndCoptic
InGujarati
InGurmukhi
InHalfwidthAndFullwidthForms
InHangulCompatibilityJamo
InHangulJamo
InHangulSyllables
InHanunoo
InHebrew
InHighPrivateUseSurrogates
InHighSurrogates
InHiragana
InIPAExtensions
InIdeographicDescriptionCharacters
InKanbun
InKangxiRadicals
InKannada
InKatakana
InKatakanaPhoneticExtensions
InKhmer
InLao
InLatin1Supplement
InLatinExtendedA
InLatinExtendedAdditional
InLatinExtendedB
InLetterlikeSymbols
InLowSurrogates
InMalayalam
InMathematicalAlphanumericSymbols
InMathematicalOperators
InMiscellaneousMathematicalSymbolsA
InMiscellaneousMathematicalSymbolsB
InMiscellaneousSymbols
InMiscellaneousTechnical
InMongolian
InMusicalSymbols
InMyanmar
InNumberForms
InOgham
InOldItalic
InOpticalCharacterRecognition
InOriya
InPrivateUseArea
InRunic
InSinhala
InSmallFormVariants
InSpacingModifierLetters
InSpecials
InSuperscriptsAndSubscripts
InSupplementalArrowsA
InSupplementalArrowsB
InSupplementalMathematicalOperators
InSupplementaryPrivateUseAreaA
InSupplementaryPrivateUseAreaB
InSyriac
InTagalog
InTagbanwa
InTags
InTamil
InTelugu
InThaana
InThai
InTibetan
InUnifiedCanadianAboriginalSyllabics
InVariationSelectors
InYiRadicals
InYiSyllables

User-Defined Character Properties

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

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

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
    }

You can also define your own mappings to be used in the lc(), lcfirst(), uc(), and ucfirst() (or their string-inlined versions). The principle is the same: define 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 property tests and mappings: they will be used only if the scalar has been marked as having Unicode characters. Old byte-style strings will not be affected.

Character Encodings for Input and Output

See Encode.

Unicode Regular Expression Support Level

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

Unicode Encodings

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

Security Implications of Unicode

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:

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?

Forcing Unicode in Perl (Or Unforcing Unicode in Perl)

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

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

Using Unicode in XS

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

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.

Interaction with Extensions

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

So if you're working with Unicode data, consult the documentation of every module you're using if there are any issues with Unicode data exchange. If the documentation does not talk about Unicode at all, suspect the worst and probably look at the source to learn how the module is implemented. Modules written completely in Perl shouldn't cause problems. Modules that directly or indirectly access code written in other programming languages are at risk.

For affected functions, the simple strategy to avoid data corruption is to always make the encoding of the exchanged data explicit. Choose an encoding that you know the extension can handle. Convert arguments passed to the extensions to that encoding and convert results back from that encoding. Write wrapper functions that do the conversions for you, so you can later change the functions when the extension catches up.

To provide an example, let's say the popular Foo::Bar::escape_html function doesn't deal with Unicode data yet. The wrapper function would convert the argument to raw UTF-8 and convert the result back to Perl's internal representation like so:

sub my_escape_html ($) {
  my($what) = shift;
  return unless defined $what;
  Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($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).

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.

SEE ALSO

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