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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.

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

Also, the use of Unicode may present security issues that aren't obvious. Read Unicode Security Considerations.

Safest if you "use feature 'unicode_strings'"

In order to preserve backward compatibility, Perl does not turn on full internal Unicode support unless the pragma use feature 'unicode_strings' is specified. (This is automatically selected if you use use 5.012 or higher.) Failure to do this can trigger unexpected surprises. See "The "Unicode Bug"" below.

This pragma doesn't affect I/O, and there are still several places where Unicode isn't fully supported, such as in filenames.

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 ":encoding(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;.

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.

Starting in Perl 5.14, Perl-level operations work with characters rather than bytes within the scope of a use feature 'unicode_strings' (or equivalently use 5.012 or higher). (This is not true if bytes have been explicitly requested by use bytes, nor necessarily true for interactions with the platform's operating system.)

For earlier Perls, and when unicode_strings is not in effect, Perl provides a fairly safe environment that can handle both types of semantics in 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.

When use locale is in effect (which overrides use feature 'unicode_strings' in the same scope), Perl uses the semantics associated with the current locale. Otherwise, Perl uses the platform's native byte semantics for characters whose code points are less than 256, and Unicode semantics for those greater than 255. On EBCDIC platforms, this is almost seamless, as the EBCDIC code pages that Perl handles are equivalent to Unicode's first 256 code points. (The exception is that EBCDIC regular expression case-insensitive matching rules are not as as robust as Unicode's.) But on ASCII platforms, Perl 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 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.

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

(The only time that Perl considers a sequence of individual code points as a single logical character is in the \X construct, already mentioned above. Therefore "character" in this discussion means a single Unicode code point.)

Very nearly all Unicode character properties are accessible through regular expressions by using the \p{} "matches property" construct and the \P{} "doesn't match property" for its negation.

For instance, \p{Uppercase} matches any single 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 property names, so \p{L} is equivalent to \pL.

More formally, \p{Uppercase} matches any single 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 several 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 that is more descriptive and hence easier to understand. Thus the "L" and "Letter" properties 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 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, white space and even hyphens can usually 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 used is in the middle of numbers, and in the Perl extension properties that begin or end with an underscore. Stricter matching cares about white space (except adjacent to non-word characters), 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}.

Almost all properties are immune to case-insensitive matching. That is, adding a /i regular expression modifier does not change what they match. There are two sets that are affected. The first set is Uppercase_Letter, Lowercase_Letter, and Titlecase_Letter, all of which match Cased_Letter under /i matching. And the second set is Uppercase, Lowercase, and Titlecase, all of which match Cased under /i matching. This set also includes its subsets PosixUpper and PosixLower both of which under /i matching match PosixAlpha. (The difference between these sets is that some things, such as Roman numerals, come in both upper and lower case so they are Cased, but aren't considered letters, so they aren't Cased_Letters.)

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
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: both are aliases for the set consisting of everything matched by Ll, Lu, and Lt.

Bidirectional Character Types

Because scripts differ in their directionality (Hebrew and Arabic are 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 many different scripts. This sentence (unless you're reading it in translation) is written in Latin, while Russian is written in Cyrillic, 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 the property can be specified 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 as well as several other blocks, 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 the digits 0-9, because those digits are shared across many scripts. The digits 0-9 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. (Note that there are several different sets of digits in Unicode that are equivalent to 0-9 and are matchable by \d in a regular expression. If they are used in a single language only, they are in that language's script. Only sets are used across several languages are in the Common script.)

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

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. Most of these are just synonyms for the Unicode ones, but some are genuine extensions, including several 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 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{ASCII}

This matches any of the 128 characters in the US-ASCII character set, which is a subset of Unicode.

\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 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 these can be variously combined 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 regular expression construct to match such sequences.

But Unicode's intent is to unify the existing character set standards and practices, and several 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 the character "LATIN CAPITAL LETTER E" followed by the character "COMBINING ACUTE ACCENT".

"LATIN CAPITAL LETTER E WITH ACUTE" is called a "pre-composed" character, and its 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 somewhat 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".

For your convenience, Perl has added the Non_Canonical decomposition type to mean any of the several 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{Posix...}

There are several of these, which are equivalents using the \p notation for Posix classes and are described in "POSIX Character Classes" in perlrecharclass.

\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 in which the decision to make them so was made. 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, as also does \p{Present_In: 3.1} and up.

\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 white 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 over 100_000 characters beyond ASCII.

\p{XPosix...}

There are several of these, which are the standard Posix classes extended to the full Unicode range. They are described in "POSIX Character Classes" in perlrecharclass.

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. However, the subroutines are passed a single parameter, which is 0 if case-sensitive matching is in effect and non-zero if caseless matching is in effect. The subroutine may return different values depending on the value of the flag, and one set of values will immutably be in effect for all case-sensitive matches, and the other set for all case-insensitive matches.

Note that if the regular expression is tainted, then Perl will die rather than calling the subroutine, where the name of the subroutine is determined by the tainted data.

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 (for serious hackers only)

This featured is deprecated and is scheduled to be removed in Perl 5.16. The CPAN module Unicode::Casing provides better functionality without the drawbacks described below.

You can define your own mappings to be used in lc(), lcfirst(), uc(), and ucfirst() (or their string-inlined versions, \L, \l, \U, and \u). The mappings are currently only valid on strings encoded in UTF-8, but see below for a partial workaround for this restriction.

The principle is similar to that of user-defined character properties: define subroutines that do the mappings. ToLower is used for lc(), \L, lcfirst(), and \l; ToTitle for ucfirst() and \u; and ToUpper for uc() and \U.

ToUpper() should look something like this:

sub ToUpper {
    return <<END;
0061\t007A\t0041
0101\t\t0100
END
}

This sample ToUpper() has the effect of mapping "a-z" to "A-Z", 0x101 to 0x100, and all other characters map to themselves. The first returned line means to map the code point at 0x61 ("a") to 0x41 ("A"), the code point at 0x62 ("b") to 0x42 ("B"), ..., 0x7A ("z") to 0x5A ("Z"). The second line maps just the code point 0x101 to 0x100. Since there are no other mappings defined, all other code points map to themselves.

This mechanism is not well behaved as far as affecting other packages and scopes. All non-threaded programs have exactly one uppercasing behavior, one lowercasing behavior, and one titlecasing behavior in effect for utf8-encoded strings for the duration of the program. Each of these behaviors is irrevocably determined the first time the corresponding function is called to change a utf8-encoded string's case. If a corresponding To- function has been defined in the package that makes that first call, the mapping defined by that function will be the mapping used for the duration of the program's execution across all packages and scopes. If no corresponding To- function has been defined in that package, the standard official mapping will be used for all packages and scopes, and any corresponding To- function anywhere will be ignored. Threaded programs have similar behavior. If the program's casing behavior has been decided at the time of a thread's creation, the thread will inherit that behavior. But, if the behavior hasn't been decided, the thread gets to decide for itself, and its decision does not affect other threads nor its creator.

As shown by the example above, 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 official mappings in the directory $Config{privlib}/unicore/To/. The mapping data is returned as the here-document. The utf8::ToSpecFoo hashes in those files 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, which is what uses the "Fold" mapping. Neither are user overridable.)

If you have many mappings to change, you can take the official mapping data, change by hand the affected code points, and place the whole thing into your subroutine. But this will only be valid on Perls that use the same Unicode version. Another option would be to have your subroutine read the official mapping files and overwrite the affected code points.

If you have only a few mappings to change, starting in 5.14 you can use the following trick, here illustrated for Turkish.

use Config;
use charnames ":full";

sub ToUpper {
    my $official = do "$Config{privlib}/unicore/To/Upper.pl";
    $utf8::ToSpecUpper{'i'} =
                       "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
    return $official;
}

This takes the official mappings and overrides just one, for "LATIN SMALL LETTER I". The keys to the hash must be the bytes that form the UTF-8 (on EBCDIC platforms, UTF-EBCDIC) of the character, as illustrated by the inverse function.

sub ToLower {
    my $official = do $lower;
    $utf8::ToSpecLower{"\xc4\xb0"} = "i";
    return $official;
}

This example is for an ASCII platform, and \xc4\xb0 is the string of bytes that together form the UTF-8 that represents \N{LATIN CAPITAL LETTER I WITH DOT ABOVE}, U+0130. You can avoid having to figure out these bytes, and at the same time make it work on all platforms by instead writing:

sub ToLower {
    my $official = do $lower;
    my $sequence = "\N{LATIN CAPITAL LETTER I WITH DOT ABOVE}";
    utf8::encode($sequence);
    $utf8::ToSpecLower{$sequence} = "i";
    return $official;
}

This works because utf8::encode() takes the single character and converts it to the sequence of bytes that constitute it. Note that we took advantage of the fact that "i" is the same in UTF-8 or UTF_EBCIDIC as not; otherwise we would have had to write

$utf8::ToSpecLower{$sequence} = "\N{LATIN SMALL LETTER I}";

in the ToLower example, and in the ToUpper example, use

my $sequence = "\N{LATIN SMALL LETTER I}";
utf8::encode($sequence);

A big caveat to the above trick and to this whole mechanism in general, is that they work only on strings encoded in UTF-8. You can partially get around this by using use subs. (But better to just convert to use Unicode::Casing.) For example: (The trick illustrated here does work in earlier releases, but only if all the characters you want to override have ordinal values of 256 or higher, or if you use the other tricks given just below.)

The mappings are in effect only for the package they are defined in, and only on scalars that have been marked as having Unicode characters, for example by using utf8::upgrade(). Although probably not advisable, you can cause the mappings to be used globally by importing into CORE::GLOBAL (see CORE).

You can partially get around the restriction that the source strings must be in utf8 by using use subs (or by importing into CORE::GLOBAL) by:

use subs qw(uc ucfirst lc lcfirst);

sub uc($) {
    my $string = shift;
    utf8::upgrade($string);
    return CORE::uc($string);
}

sub lc($) {
    my $string = shift;
    utf8::upgrade($string);

    # Unless an I is before a dot_above, it turns into a dotless i.
    # (The character class with the combining classes matches non-above
    # marks following the I.  Any number of these may be between the 'I' and
    # the dot_above, and the dot_above will still apply to the 'I'.
    use charnames ":full";
    $string =~
            s/I
              (?! [^\p{ccc=0}\p{ccc=Above}]* \N{COMBINING DOT ABOVE} )
             /\N{LATIN SMALL LETTER DOTLESS I}/gx;

    # But when the I is followed by a dot_above, remove the
    # dot_above so the end result will be i.
    $string =~ s/I
                   ([^\p{ccc=0}\p{ccc=Above}]* )
                   \N{COMBINING DOT ABOVE}
                /i$1/gx;
    return CORE::lc($string);
}

These examples (also for Turkish) make sure the input is in UTF-8, and then call the corresponding official function, which will use the ToUpper() and ToLower() functions you have defined. (For Turkish, there are other required functions: ucfirst, lcfirst, and ToTitle. These are very similar to the ones given above.)

The reason this is only a partial fix is that it doesn't affect the \l, \L, \u, and \U case-change operations in regular expressions, which still require the source to be encoded in utf8 (see "The "Unicode Bug""). (Again, use Unicode::Casing instead.)

The lc() example shows how you can add context-dependent casing. Note that context-dependent casing suffers from the problem that the string passed to the casing function may not have sufficient context to make the proper choice. Also, it will not be called for \l, \L, \u, and \U.

Character Encodings for Input and Output

See Encode.

Unicode Regular Expression Support Level

The following list of Unicode supported features for regular expressions describes all features currently directly supported by core Perl. The references to "Level N" and the section numbers refer to the Unicode Technical Standard #18, "Unicode Regular Expressions", version 13, from August 2008.

Unicode Encodings

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

Non-character code points

66 code points are set aside in Unicode as "non-character code points". These all have the Unassigned (Cn) General Category, and they never will be assigned. These are never supposed to be in legal Unicode input streams, so that code can use them as sentinels that can be mixed in with character data, and they always will be distinguishable from that data. To keep them out of Perl input streams, strict UTF-8 should be specified, such as by using the layer :encoding('UTF-8'). The non-character code points are the 32 between U+FDD0 and U+FDEF, and the 34 code points U+FFFE, U+FFFF, U+1FFFE, U+1FFFF, ... U+10FFFE, U+10FFFF. Some people are under the mistaken impression that these are "illegal", but that is not true. An application or cooperating set of applications can legally use them at will internally; but these code points are "illegal for open interchange". Therefore, Perl will not accept these from input streams unless lax rules are being used, and will warn (using the warning category "nonchar", which is a sub-category of "utf8") if an attempt is made to output them.

Beyond Unicode code points

The maximum Unicode code point is U+10FFFF. But Perl accepts code points up to the maximum permissible unsigned number available on the platform. However, Perl will not accept these from input streams unless lax rules are being used, and will warn (using the warning category "non_unicode", which is a sub-category of "utf8") if an attempt is made to operate on or output them. For example, uc(0x11_0000) will generate this warning, returning the input parameter as its result, as the upper case of every non-Unicode code point is the code point itself.

Security Implications of Unicode

Read Unicode Security Considerations. Also, note the following:

As discussed elsewhere, Perl has one foot (two hooves?) planted in each of two worlds: the old world of bytes and the new world of characters, upgrading from bytes to characters when necessary. If your legacy code does not explicitly use Unicode, no automatic switch-over to characters should happen. Characters shouldn't get downgraded to bytes, either. It is possible to accidentally mix bytes and characters, however (see perluniintro), in which case \w in regular expressions might start behaving differently (unless the /a modifier is in effect). Review your code. Use warnings and the strict pragma.

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

See "Unicode and UTF-8" in perllocale

When Unicode Does Not Happen

While Perl does have extensive ways to input and output in Unicode, and a few other "entry points" like the @ARGV array (which can sometimes be interpreted as UTF-8), there are still 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 (problematic) encoding pragma has been used.

One reason that Perl does not attempt to resolve the role of Unicode in these situations 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 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 on ASCII platforms with the Unicode code points 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, unless use feature 'unicode_strings' is specified. (The lesson here is to specify unicode_strings to avoid the headaches.)

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.

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'
    no feature 'unicode_strings';
    $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.

Starting in Perl 5.14, use feature 'unicode_strings' can be used to cause Perl to use Unicode semantics on all string operations within the scope of the feature subpragma. Regular expressions compiled in its scope retain that behavior even when executed or compiled into larger regular expressions outside the scope. (The pragma does not, however, affect the quotemeta behavior. Nor does it affect the deprecated user-defined case changing operations--these still require a UTF-8 encoded string to operate.)

In Perl 5.12, the subpragma affected casing changes, but not regular expressions. See "lc" in perlfunc for details on how this pragma works in combination with various others for casing.

For earlier Perls, or when a string is passed to a function outside the subpragma's scope, 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 desired version of Unicode from the Unicode web site http://www.unicode.org). These should replace the existing files in lib/unicore in the Perl source tree. Follow the instructions in README.perl in that directory to change some of their names, and then build perl (see INSTALL).

It is even possible to copy the built files 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

See "Unicode and UTF-8" in perllocale

Problems with characters in the Latin-1 Supplement range

See "The "Unicode Bug""

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 recognize that 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 able 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 are hundreds of Unicode characters matching Nd compared with the 10 ASCII characters matching d).

Problems on EBCDIC platforms

There are several 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).