perlunicode - Unicode support in Perl

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 the open manpage.

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

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 the utf8 manpage.

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

BOM-marked scripts and UTF-16 scripts autodetected
If a Perl script begins marked with the Unicode BOM (UTF-16LE, UTF16-BE, or UTF-8), or if the script looks like non-BOM-marked UTF-16 of either endianness, Perl will correctly read in the script as Unicode. (BOMless UTF-8 cannot be effectively recognized or differentiated from ISO 8859-1 or other eight-bit encodings.)

use encoding needed to upgrade non-Latin-1 byte strings
By default, there is a fundamental asymmetry in Perl's unicode model: implicit upgrading from byte strings to Unicode strings assumes that they were encoded in ISO 8859-1 (Latin-1), but Unicode strings are downgraded with UTF-8 encoding. This happens because the first 256 codepoints in Unicode happens to agree with Latin-1.

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 the bytes manpage.

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 the utf8 manpage.

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 the encoding manpage.

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 the perluniintro manpage 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 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:

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

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

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

You could also have used the existing block property names:

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

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

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

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

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

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

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

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

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 the Encode manpage.

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 the perlebcdic manpage 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 the perlguts manpage for an explanation about Unicode at the XS level, and the perlapi manpage for the API details.

For more information, see the perlapi manpage, 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

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

 perlunicode - Unicode support in Perl