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.

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

Input and Output Layers

Perl knows when a filehandle uses Perl's internal Unicode encodings (UTF-8, or UTF-EBCDIC if in EBCDIC) if the filehandle is opened with the ":utf8" layer. Other encodings can be converted to Perl's encoding on input or from Perl's encoding on output by use of the ":encoding(...)" layer. See open.

To indicate that Perl source itself is in UTF-8, use use utf8;.

Regular Expressions

The regular expression compiler produces polymorphic opcodes. That is, the pattern adapts to the data and automatically switches to the Unicode character scheme when presented with data that is internally encoded in UTF-8 -- or instead uses a traditional byte scheme when presented with byte data.

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

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

BOM-marked scripts and UTF-16 scripts autodetected

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

use encoding needed to upgrade non-Latin-1 byte strings

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

See "Byte and Character Semantics" for more details.

Byte and Character Semantics

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

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

However, as an interim compatibility measure, Perl aims to provide a safe migration path from byte semantics to character semantics for programs. For operations where Perl can unambiguously decide that the input data are characters, Perl switches to character semantics. For operations where this determination cannot be made without additional information from the user, Perl decides in favor of compatibility and chooses to use byte semantics.

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.

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:

Unicode Character Properties

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

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

The official Unicode script and block names have spaces and dashes as separators, but for convenience you can use dashes, spaces, or underbars, and case is unimportant. It is recommended, however, that for consistency you use the following naming: the official Unicode script, property, or block name (see below for the additional rules that apply to block names) with whitespace and dashes removed, and the words "uppercase-first-lowercase-rest". Latin-1 Supplement thus becomes Latin1Supplement.

You can also use negation in both \p{} and \P{} by introducing a caret (^) between the first brace and the property name: \p{^Tamil} is equal to \P{Tamil}.

NOTE: the properties, scripts, and blocks listed here are as of Unicode 5.0.0 in July 2006.

General Category

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

Short       Long

L           Letter
LC          CasedLetter
Lu          UppercaseLetter
Ll          LowercaseLetter
Lt          TitlecaseLetter
Lm          ModifierLetter
Lo          OtherLetter

M           Mark
Mn          NonspacingMark
Mc          SpacingMark
Me          EnclosingMark

N           Number
Nd          DecimalNumber
Nl          LetterNumber
No          OtherNumber

P           Punctuation
Pc          ConnectorPunctuation
Pd          DashPunctuation
Ps          OpenPunctuation
Pe          ClosePunctuation
Pi          InitialPunctuation
            (may behave like Ps or Pe depending on usage)
Pf          FinalPunctuation
            (may behave like Ps or Pe depending on usage)
Po          OtherPunctuation

S           Symbol
Sm          MathSymbol
Sc          CurrencySymbol
Sk          ModifierSymbol
So          OtherSymbol

Z           Separator
Zs          SpaceSeparator
Zl          LineSeparator
Zp          ParagraphSeparator

C           Other
Cc          Control
Cf          Format
Cs          Surrogate   (not usable)
Co          PrivateUse
Cn          Unassigned

Single-letter properties match all characters in any of the two-letter sub-properties starting with the same letter. LC and L& are special cases, which are aliases for the set of Ll, Lu, and Lt.

Because Perl hides the need for the user to understand the internal representation of Unicode characters, there is no need to implement the somewhat messy concept of surrogates. Cs is therefore not supported.

Bidirectional Character Types

Because scripts differ in their directionality--Hebrew is written right to left, for example--Unicode supplies these properties in the BidiClass class:

Property    Meaning

L           Left-to-Right
LRE         Left-to-Right Embedding
LRO         Left-to-Right Override
R           Right-to-Left
AL          Right-to-Left Arabic
RLE         Right-to-Left Embedding
RLO         Right-to-Left Override
PDF         Pop Directional Format
EN          European Number
ES          European Number Separator
ET          European Number Terminator
AN          Arabic Number
CS          Common Number Separator
NSM         Non-Spacing Mark
BN          Boundary Neutral
B           Paragraph Separator
S           Segment Separator
WS          Whitespace
ON          Other Neutrals

For example, \p{BidiClass:R} matches characters that are normally written right to left.

Scripts

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

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

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

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

and there are further derived properties:

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

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

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

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

Common      =  Any character (or unassigned code point)
               not explicitly assigned to a script
Use of "Is" Prefix

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

Blocks

In addition to scripts, Unicode also defines blocks of characters. The difference between scripts and blocks is that the concept of scripts is closer to natural languages, while the concept of blocks is more of an artificial grouping based on groups of 256 Unicode characters. For example, the Latin script contains letters from many blocks but does not contain all the characters from those blocks. It does not, for example, contain digits, because digits are shared across many scripts. Digits and similar groups, like punctuation, are in a category called Common.

For more about scripts, see the UAX#24 "Script Names":

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

For more about blocks, see:

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

Block names are given with the In prefix. For example, the Katakana block is referenced via \p{InKatakana}. The In prefix may be omitted if there is no naming conflict with a script or any other property, but it is recommended that In always be used for block tests to avoid confusion.

These block names are supported:

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

User-Defined Character Properties

You can define your own character properties by defining subroutines whose names begin with "In" or "Is". The subroutines can be defined in any package. The user-defined properties can be used in the regular expression \p and \P constructs; if you are using a user-defined property from a package other than the one you are in, you must specify its package in the \p or \P construct.

# assuming property IsForeign defined in Lang::
package main;  # property package name required
if ($txt =~ /\p{Lang::IsForeign}+/) { ... }

package Lang;  # property package name not required
if ($txt =~ /\p{IsForeign}+/) { ... }

Note that the effect is compile-time and immutable once defined.

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

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

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

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

You could also have used the existing block property names:

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

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

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

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

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

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

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

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

User-Defined Case Mappings

You can also define your own mappings to be used in the lc(), lcfirst(), uc(), and ucfirst() (or their string-inlined versions). The principle is similar to that of user-defined character properties: to define subroutines in the main package with names like ToLower (for lc() and lcfirst()), ToTitle (for the first character in ucfirst()), and ToUpper (for uc(), and the rest of the characters in ucfirst()).

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

sub ToUpper {
    return <<END;
0061\t0063\t0041
END
}

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

If there is no source range to speak of, that is, the mapping is from a single character to another single character, leave the end of the source range empty, but the two tabulator characters are still needed. For example:

sub ToLower {
    return <<END;
0041\t\t0061
END
}

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

(For serious hackers only) If you want to introspect the default mappings, you can find the data in the directory $Config{privlib}/unicore/To/. The mapping data is returned as the here-document, and the utf8::ToSpecFoo are special exception mappings derived from <$Config{privlib}>/unicore/SpecialCasing.txt. The Digit and Fold mappings that one can see in the directory are not directly user-accessible, one can use either the Unicode::UCD module, or just match case-insensitively (that's when the Fold mapping is used).

A final note on the user-defined case mappings: they will be used only if the scalar has been marked as having Unicode characters. Old byte-style strings will not be affected.

Character Encodings for Input and Output

See Encode.

Unicode Regular Expression Support Level

The following list of Unicode support for regular expressions describes all the features currently supported. The references to "Level N" and the section numbers refer to the Unicode Technical Standard #18, "Unicode Regular Expressions", version 11, in May 2005.

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 UTF8 flag and act accordingly. If the extension doesn't know about the flag, it's likely that the extension will return incorrectly-flagged data.

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

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

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

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

Sometimes, when the extension does not convert data but just stores and retrieves them, you will be in a position to use the otherwise dangerous Encode::_utf8_on() function. Let's say the popular Foo::Bar extension, written in C, provides a param method that lets you store and retrieve data according to these prototypes:

$self->param($name, $value);            # set a scalar
$value = $self->param($name);           # retrieve a scalar

If it does not yet provide support for any encoding, one could write a derived class with such a param method:

sub param {
  my($self,$name,$value) = @_;
  utf8::upgrade($name);     # make sure it is UTF-8 encoded
  if (defined $value) {
    utf8::upgrade($value);  # make sure it is UTF-8 encoded
    return $self->SUPER::param($name,$value);
  } else {
    my $ret = $self->SUPER::param($name);
    Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
    return $ret;
  }
}

Some extensions provide filters on data entry/exit points, such as DB_File::filter_store_key and family. Look out for such filters in the documentation of your extensions, they can make the transition to Unicode data much easier.

Speed

Some functions are slower when working on UTF-8 encoded strings than on byte encoded strings. All functions that need to hop over characters such as length(), substr() or index(), or matching regular expressions can work much faster when the underlying data are byte-encoded.

In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a caching scheme was introduced which will hopefully make the slowness somewhat less spectacular, at least for some operations. In general, operations with UTF-8 encoded strings are still slower. As an example, the Unicode properties (character classes) like \p{Nd} are known to be quite a bit slower (5-20 times) than their simpler counterparts like \d (then again, there 268 Unicode characters matching Nd compared with the 10 ASCII characters matching d).

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

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