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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 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, and absent a use feature 'unicode_strings' pragma, 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 a 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 use feature 'unicode_strings' pragma is intended to always, regardless of platform, force Unicode semantics in a particular lexical scope. In release 5.12, it is partially implemented, applying only to case changes. See "The "Unicode Bug"" below.

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, and the use feature 'unicode_strings' pragma to force Unicode semantics on byte data (though in 5.12 it isn't fully implemented).

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. You can choose to be warned when this happens. See encoding::warnings.

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

Most Unicode character properties are accessible by using regular expressions. They are used like character classes via the \p{} "matches property" construct and the \P{} negation, "doesn't match property".

For instance, \p{Uppercase} matches any character with the Unicode "Uppercase" property, while \p{L} matches any character with a General_Category of "L" (letter) property. Brackets are not required for single letter properties, so \p{L} is equivalent to \pL.

More formally, \p{Uppercase} matches any character whose Unicode Uppercase property value is True, and \P{Uppercase} matches any character whose Uppercase property value is False, and they could have been written as \p{Uppercase=True} and \p{Uppercase=False}, respectively

This formality is needed when properties are not binary, that is if they can take on more values than just True and False. For example, the Bidi_Class (see "Bidirectional Character Types" below), can take on a number of different values, such as Left, Right, Whitespace, and others. To match these, one needs to specify the property name (Bidi_Class), and the value being matched against (Left, Right, etc.). This is done, as in the examples above, by having the two components separated by an equal sign (or interchangeably, a colon), like \p{Bidi_Class: Left}.

All Unicode-defined character properties may be written in these compound forms of \p{property=value} or \p{property:value}, but Perl provides some additional properties that are written only in the single form, as well as single-form short-cuts for all binary properties and certain others described below, in which you may omit the property name and the equals or colon separator.

Most Unicode character properties have at least two synonyms (or aliases if you prefer), a short one that is easier to type, and a longer one which is more descriptive and hence it is easier to understand what it means. Thus the "L" and "Letter" above are equivalent and can be used interchangeably. Likewise, "Upper" is a synonym for "Uppercase", and we could have written \p{Uppercase} equivalently as \p{Upper}. Also, there are typically various synonyms for the values the property can be. For binary properties, "True" has 3 synonyms: "T", "Yes", and "Y"; and "False has correspondingly "F", "No", and "N". But be careful. A short form of a value for one property may not mean the same thing as the same short form for another. Thus, for the General_Category property, "L" means "Letter", but for the Bidi_Class property, "L" means "Left". A complete list of properties and synonyms is in perluniprops.

Upper/lower case differences in the property names and values are irrelevant, thus \p{Upper} means the same thing as \p{upper} or even \p{UpPeR}. Similarly, you can add or subtract underscores anywhere in the middle of a word, so that these are also equivalent to \p{U_p_p_e_r}. And white space is irrelevant adjacent to non-word characters, such as the braces and the equals or colon separators so \p{ Upper } and \p{ Upper_case : Y } are equivalent to these as well. In fact, in most cases, white space and even hyphens can be added or deleted anywhere. So even \p{ Up-per case = Yes} is equivalent. All this is called "loose-matching" by Unicode. The few places where stricter matching is employed is in the middle of numbers, and the Perl extension properties that begin or end with an underscore. Stricter matching cares about white space (except adjacent to the non-word characters) and hyphens, and non-interior underscores.

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}.

General_Category

Every Unicode character is assigned a general category, which is the "most usual categorization of a character" (from http://www.unicode.org/reports/tr44).

The compound way of writing these is like \p{General_Category=Number} (short, \p{gc:n}). But Perl furnishes shortcuts in which everything up through the equal or colon separator is omitted. So you can instead just write \pN.

Here are the short and long forms of the General Category properties:

    Short       Long

    L           Letter
    LC, L&      Cased_Letter (that is: [\p{Ll}\p{Lu}\p{Lt}])
    Lu          Uppercase_Letter
    Ll          Lowercase_Letter
    Lt          Titlecase_Letter
    Lm          Modifier_Letter
    Lo          Other_Letter

    M           Mark
    Mn          Nonspacing_Mark
    Mc          Spacing_Mark
    Me          Enclosing_Mark

    N           Number
    Nd          Decimal_Number (also Digit)
    Nl          Letter_Number
    No          Other_Number

    P           Punctuation (also Punct)
    Pc          Connector_Punctuation
    Pd          Dash_Punctuation
    Ps          Open_Punctuation
    Pe          Close_Punctuation
    Pi          Initial_Punctuation
                (may behave like Ps or Pe depending on usage)
    Pf          Final_Punctuation
                (may behave like Ps or Pe depending on usage)
    Po          Other_Punctuation

    S           Symbol
    Sm          Math_Symbol
    Sc          Currency_Symbol
    Sk          Modifier_Symbol
    So          Other_Symbol

    Z           Separator
    Zs          Space_Separator
    Zl          Line_Separator
    Zp          Paragraph_Separator

    C           Other
    Cc          Control (also Cntrl)
    Cf          Format
    Cs          Surrogate   (not usable)
    Co          Private_Use
    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 Bidi_Class class:

    Property    Meaning

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

This property is always written in the compound form. For example, \p{Bidi_Class:R} matches characters that are normally written right to left.

Scripts

The world's languages are written in a number of scripts. This sentence (unless you're reading it in translation) is written in Latin, while Russian is written in Cyrllic, and Greek is written in, well, Greek; Japanese mainly in Hiragana or Katakana. There are many more.

The Unicode Script property gives what script a given character is in, and can be matched with the compound form like \p{Script=Hebrew} (short: \p{sc=hebr}). Perl furnishes shortcuts for all script names. You can omit everything up through the equals (or colon), and simply write \p{Latin} or \P{Cyrillic}.

A complete list of scripts and their shortcuts is in perluniprops.

Use of "Is" Prefix

For backward compatibility (with Perl 5.6), all properties mentioned so far may have Is or Is_ prepended to their name, so \P{Is_Lu}, for example, is equal to \P{Lu}, and \p{IsScript:Arabic} is equal to \p{Arabic}.

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 Unicode characters with consecutive ordinal values. For example, the "Basic Latin" block is all characters whose ordinals are between 0 and 127, inclusive, in other words, the ASCII characters. The "Latin" script contains some letters from this block as well as several more, like "Latin-1 Supplement", "Latin Extended-A", etc., but it 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 the script called Common. There is also a script called Inherited for characters that modify other characters, and inherit the script value of the controlling character.

For more about scripts versus blocks, see UAX#24 "Unicode Script Property": http://www.unicode.org/reports/tr24

The Script property is likely to be the one you want to use when processing natural language; the Block property may be useful in working with the nuts and bolts of Unicode.

Block names are matched in the compound form, like \p{Block: Arrows} or \p{Blk=Hebrew}. Unlike most other properties only a few block names have a Unicode-defined short name. But Perl does provide a (slight) shortcut: You can say, for example \p{In_Arrows} or \p{In_Hebrew}. For backwards compatibility, the In prefix may be omitted if there is no naming conflict with a script or any other property, and you can even use an Is prefix instead in those cases. But it is not a good idea to do this, for a couple reasons:

  1. It is confusing. There are many naming conflicts, and you may forget some. For example, \p{Hebrew} means the script Hebrew, and NOT the block Hebrew. But would you remember that 6 months from now?
  2. It is unstable. A new version of Unicode may pre-empt the current meaning by creating a property with the same name. There was a time in very early Unicode releases when \p{Hebrew} would have matched the block Hebrew; now it doesn't.

Some people just prefer to always use \p{Block: foo} and \p{Script: bar} instead of the shortcuts, for clarity, and because they can't remember the difference between 'In' and 'Is' anyway (or aren't confident that those who eventually will read their code will know).

A complete list of blocks and their shortcuts is in perluniprops.

Other Properties

There are many more properties than the very basic ones described here. A complete list is in perluniprops.

Unicode defines all its properties in the compound form, so all single-form properties are Perl extensions. A number of these are just synonyms for the Unicode ones, but some are genunine extensions, including a couple that are in the compound form. And quite a few of these are actually recommended by Unicode (in http://www.unicode.org/reports/tr18).

This section gives some details on all the extensions that aren't synonyms for compound-form Unicode properties (for those, you'll have to refer to the Unicode Standard.

\p{All}

This matches any of the 1_114_112 Unicode code points. It is a synonym for \p{Any}.

\p{Alnum}

This matches any \p{Alphabetic} or \p{Decimal_Number} character.

\p{Any}

This matches any of the 1_114_112 Unicode code points. It is a synonym for \p{All}.

\p{Assigned}

This matches any assigned code point; that is, any code point whose general category is not Unassigned (or equivalently, not Cn).

\p{Blank}

This is the same as \h and \p{HorizSpace}: A character that changes the spacing horizontally.

\p{Decomposition_Type: Non_Canonical} (Short: \p{Dt=NonCanon})

Matches a character that has a non-canonical decomposition.

To understand the use of this rarely used property=value combination, it is necessary to know some basics about decomposition. Consider a character, say H. It could appear with various marks around it, such as an acute accent, or a circumflex, or various hooks, circles, arrows, etc., above, below, to one side and/or the other, etc. There are many possibilities among the world's languages. The number of combinations is astronomical, and if there were a character for each combination, it would soon exhaust Unicode's more than a million possible characters. So Unicode took a different approach: there is a character for the base H, and a character for each of the possible marks, and they can be combined variously to get a final logical character. So a logical character--what appears to be a single character--can be a sequence of more than one individual characters. This is called an "extended grapheme cluster". (Perl furnishes the \X construct to match such sequences.)

But Unicode's intent is to unify the existing character set standards and practices, and a number of pre-existing standards have single characters that mean the same thing as some of these combinations. An example is ISO-8859-1, which has quite a few of these in the Latin-1 range, an example being "LATIN CAPITAL LETTER E WITH ACUTE". Because this character was in this pre-existing standard, Unicode added it to its repertoire. But this character is considered by Unicode to be equivalent to the sequence consisting of first the character "LATIN CAPITAL LETTER E", then the character "COMBINING ACUTE ACCENT".

"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and the equivalence with the sequence is called canonical equivalence. All pre-composed characters are said to have a decomposition (into the equivalent sequence) and the decomposition type is also called canonical.

However, many more characters have a different type of decomposition, a "compatible" or "non-canonical" decomposition. The sequences that form these decompositions are not considered canonically equivalent to the pre-composed character. An example, again in the Latin-1 range, is the "SUPERSCRIPT ONE". It is kind of like a regular digit 1, but not exactly; its decomposition into the digit 1 is called a "compatible" decomposition, specifically a "super" decomposition. There are several such compatibility decompositions (see http://www.unicode.org/reports/tr44), including one called "compat" which means some miscellaneous type of decomposition that doesn't fit into the decomposition categories that Unicode has chosen.

Note that most Unicode characters don't have a decomposition, so their decomposition type is "None".

Perl has added the Non_Canonical type, for your convenience, to mean any of the compatibility decompositions.

\p{Graph}

Matches any character that is graphic. Theoretically, this means a character that on a printer would cause ink to be used.

\p{HorizSpace}

This is the same as \h and \p{Blank}: A character that changes the spacing horizontally.

\p{In=*}

This is a synonym for \p{Present_In=*}

\p{PerlSpace}

This is the same as \s, restricted to ASCII, namely [ \f\n\r\t].

Mnemonic: Perl's (original) space

\p{PerlWord}

This is the same as \w, restricted to ASCII, namely [A-Za-z0-9_]

Mnemonic: Perl's (original) word.

\p{PosixAlnum}

This matches any alphanumeric character in the ASCII range, namely [A-Za-z0-9].

\p{PosixAlpha}

This matches any alphabetic character in the ASCII range, namely [A-Za-z].

\p{PosixBlank}

This matches any blank character in the ASCII range, namely [ \t].

\p{PosixCntrl}

This matches any control character in the ASCII range, namely [\x00-\x1F\x7F]

\p{PosixDigit}

This matches any digit character in the ASCII range, namely [0-9].

\p{PosixGraph}

This matches any graphical character in the ASCII range, namely [\x21-\x7E].

\p{PosixLower}

This matches any lowercase character in the ASCII range, namely [a-z].

\p{PosixPrint}

This matches any printable character in the ASCII range, namely [\x20-\x7E]. These are the graphical characters plus SPACE.

\p{PosixPunct}

This matches any punctuation character in the ASCII range, namely [\x21-\x2F\x3A-\x40\x5B-\x60\x7B-\x7E]. These are the graphical characters that aren't word characters. Note that the Posix standard includes in its definition of punctuation, those characters that Unicode calls "symbols."

\p{PosixSpace}

This matches any space character in the ASCII range, namely [ \f\n\r\t\x0B] (the last being a vertical tab).

\p{PosixUpper}

This matches any uppercase character in the ASCII range, namely [A-Z].

\p{Present_In: *} (Short: \p{In=*})

This property is used when you need to know in what Unicode version(s) a character is.

The "*" above stands for some two digit Unicode version number, such as 1.1 or 4.0; or the "*" can also be Unassigned. This property will match the code points whose final disposition has been settled as of the Unicode release given by the version number; \p{Present_In: Unassigned} will match those code points whose meaning has yet to be assigned.

For example, U+0041 "LATIN CAPITAL LETTER A" was present in the very first Unicode release available, which is 1.1, so this property is true for all valid "*" versions. On the other hand, U+1EFF was not assigned until version 5.1 when it became "LATIN SMALL LETTER Y WITH LOOP", so the only "*" that would match it are 5.1, 5.2, and later.

Unicode furnishes the Age property from which this is derived. The problem with Age is that a strict interpretation of it (which Perl takes) has it matching the precise release a code point's meaning is introduced in. Thus U+0041 would match only 1.1; and U+1EFF only 5.1. This is not usually what you want.

Some non-Perl implementations of the Age property may change its meaning to be the same as the Perl Present_In property; just be aware of that.

Another confusion with both these properties is that the definition is not that the code point has been assigned, but that the meaning of the code point has been determined. This is because 66 code points will always be unassigned, and, so the Age for them is the Unicode version the decision to make them so was made in. For example, U+FDD0 is to be permanently unassigned to a character, and the decision to do that was made in version 3.1, so \p{Age=3.1} matches this character and \p{Present_In: 3.1} and up matches as well.

\p{Print}

This matches any character that is graphical or blank, except controls.

\p{SpacePerl}

This is the same as \s, including beyond ASCII.

Mnemonic: Space, as modified by Perl. (It doesn't include the vertical tab which both the Posix standard and Unicode consider to be space.)

\p{VertSpace}

This is the same as \v: A character that changes the spacing vertically.

\p{Word}

This is the same as \w, including beyond ASCII.

User-Defined Character Properties

You can define your own binary 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 Is_Foreign 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 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 to be two hexadecimal numbers separated by two tabulators: the two numbers being, respectively, the source code point and the destination code point. For example:

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

defines an uc() mapping that causes only the character "a" to be mapped to "A"; all other characters will remain unchanged.

(For serious hackers only) The above means you have to furnish a complete mapping; you can't just override a couple of characters and leave the rest unchanged. You can find all the mappings 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).

The mappings will only take effect on scalars that have been marked as having Unicode characters, for example by using utf8::upgrade(). Old byte-style strings are not affected.

The mappings are in effect for the package they are defined in.

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

Read Unicode Security Considerations. Also, note the following:

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. Also, see "The "Unicode Bug"". 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 these 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?

The "Unicode Bug"

The term, the "Unicode bug" has been applied to an inconsistency with the Unicode characters whose ordinals are in the Latin-1 Supplement block, that is, between 128 and 255. 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 have the same semantics 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. (On EBCDIC platforms, the behavior may be different from this, depending on the underlying C language library functions.)

The behavior is known to have effects on these areas:

This behavior 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. As an example, consider the following program and its output:

 $ perl -le'
     $s1 = "\xC2";
     $s2 = "\x{2660}";
     for ($s1, $s2, $s1.$s2) {
         print /\w/ || 0;
     }
 '
 0
 0
 1

If there's no \w in s1 or in s2, why does their concatenation have one?

This anomaly stems from Perl's attempt to not disturb older programs that didn't use Unicode, and hence had no semantics for characters outside of the ASCII range (except in a locale), along with Perl's desire to add Unicode support seamlessly. The result wasn't seamless: these characters were orphaned.

Work is being done to correct this, but only some of it was complete in time for the 5.12 release. What has been finished is the important part of the case changing component. Due to concerns, and some evidence, that older code might have come to rely on the existing behavior, the new behavior must be explicitly enabled by the feature unicode_strings in the feature pragma, even though no new syntax is involved.

See "lc" in perlfunc for details on how this pragma works in combination with various others for casing. Even though the pragma only affects casing operations in the 5.12 release, it is planned to have it affect all the problematic behaviors in later releases: you can't have one without them all.

In the meantime, a workaround is to always call utf8::upgrade($string), or to use the standard module Encode. Also, a scalar that has any characters whose ordinal is above 0x100, or which were specified using either of the \N{...} notations will automatically have character semantics.

Forcing Unicode in Perl (Or Unforcing Unicode in Perl)

Sometimes (see "When Unicode Does Not Happen" or "The "Unicode Bug"") 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.

Calling either function on a string that already is in the desired state is a no-op.

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.

Hacking Perl to work on earlier Unicode versions (for very serious hackers only)

Perl by default comes with the latest supported Unicode version built in, but you can change to use any earlier one.

Download the files in the version of Unicode that you want from the Unicode web site http://www.unicode.org). These should replace the existing files in \$Config{privlib}/unicore. (\%Config is available from the Config module.) Follow the instructions in README.perl in that directory to change some of their names, and then run make.

It is even possible to download them to a different directory, and then change utf8_heavy.pl in the directory \$Config{privlib} to point to the new directory, or maybe make a copy of that directory before making the change, and using @INC or the -I run-time flag to switch between versions at will (but because of caching, not in the middle of a process), but all this is beyond the scope of these instructions.

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 in the Latin-1 Supplement range

See "The "Unicode Bug""

Problems with case-insensitive regular expression matching

There are problems with case-insensitive matches, including those involving character classes (enclosed in [square brackets]), characters whose fold is to multiple characters (such as the single character LATIN SMALL LIGATURE FFL matches case-insensitively with the 3-character string ffl), and characters in the Latin-1 Supplement.

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).

Problems on EBCDIC platforms

There are a number of known problems with Perl on EBCDIC platforms. If you want to use Perl there, send email to perlbug@perl.org.

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 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, perluniprops, Encode, open, utf8, bytes, perlretut, "${^UNICODE}" in perlvar http://www.unicode.org/reports/tr44).

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