PERLUNICODE(1) Perl Programmers Reference Guide PERLUNICODE(1)
NAME
perlunicode - Unicode support in Perl
DESCRIPTION
Important Caveats
Unicode support is an extensive requirement. While Perl does not implement the Unicode standard or
the accompanying technical reports from cover to cover, Perl does support many Unicode features.
Input and Output Layers
Perl knows when a filehandle uses Perl's internal Unicode encodings (UTF-8, or UTF-EBCDIC if in
EBCDIC) if the filehandle is opened with the ":utf8" layer. Other encodings can be converted to
Perl's encoding on input or from Perl's encoding on output by use of the ":encoding(...)" layer.
See open.
To indicate that Perl source itself is using a particular encoding, see encoding.
Regular Expressions
The regular expression compiler produces polymorphic opcodes. That is, the pattern adapts to the
data and automatically switches to the Unicode character scheme when presented with Unicode
data--or instead uses a traditional byte scheme when presented with byte data.
"use utf8" still needed to enable UTF-8/UTF-EBCDIC in scripts
As a compatibility measure, the "use utf8" pragma must be explicitly included to enable recogni-tion recognition
tion of UTF-8 in the Perl scripts themselves (in string or regular expression literals, or in
identifier names) on ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based machines.
These are the only times when an explicit "use utf8" is needed. See utf8.
You can also use the "encoding" pragma to change the default encoding of the data in your script;
see encoding.
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 Uni-code Unicode
code strings are downgraded with UTF-8 encoding. This happens because the first 256 codepoints
in Unicode happens to agree with Latin-1.
If you wish to interpret byte strings as UTF-8 instead, use the "encoding" pragma:
use encoding 'utf8';
See "Byte and Character Semantics" for more details.
Byte and Character Semantics
Beginning with version 5.6, Perl uses logically-wide characters to represent strings internally.
In future, Perl-level operations will be expected to work with characters rather than bytes.
However, as an interim compatibility measure, Perl aims to provide a safe migration path from byte
semantics to character semantics for programs. For operations where Perl can unambiguously decide
that the input data are characters, Perl switches to character semantics. For operations where this
determination cannot be made without additional information from the user, Perl decides in favor of
compatibility and chooses to use byte semantics.
This behavior preserves compatibility with earlier versions of Perl, which allowed byte semantics in
Perl operations only if none of the program's inputs were marked as being as source of Unicode char-acter character
acter data. Such data may come from filehandles, from calls to external programs, from information
provided by the system (such as %ENV), or from literals and constants in the source text.
The "bytes" pragma will always, regardless of platform, force byte semantics in a particular lexical
scope. See bytes.
The "utf8" pragma is primarily a compatibility device that enables recognition of UTF-(8|EBCDIC) in
literals encountered by the parser. Note that this pragma is only required while Perl defaults to
byte semantics; when character semantics become the default, this pragma may become a no-op. See
utf8.
Unless explicitly stated, Perl operators use character semantics for Unicode data and byte semantics
for non-Unicode data. The decision to use character semantics is made transparently. If input data
comes from a Unicode source--for example, if a character encoding layer is added to a filehandle or a
literal Unicode string constant appears in a program--character semantics apply. Otherwise, byte
semantics are in effect. The "bytes" pragma should be used to force byte semantics on Unicode data.
If strings operating under byte semantics and strings with Unicode character data are concatenated,
the new string will be created by decoding the byte strings as ISO 8859-1 (Latin-1), even if the old
Unicode string used EBCDIC. This translation is done without regard to the system's native 8-bit
encoding. To change this for systems with non-Latin-1 and non-EBCDIC native encodings, use the
"encoding" pragma. See encoding.
Under character semantics, many operations that formerly operated on bytes now operate on characters.
A character in Perl is logically just a number ranging from 0 to 2**31 or so. Larger characters may
encode into longer sequences of bytes internally, but this internal detail is mostly hidden for Perl
code. See perluniintro for more.
Effects of Character Semantics
Character semantics have the following effects:
Strings--including hash keys--and regular expression patterns may contain characters that have an
ordinal value larger than 255.
If you use a Unicode editor to edit your program, Unicode characters may occur directly within
the literal strings in one of the various Unicode encodings (UTF-8, UTF-EBCDIC, UCS-2, etc.), but
will be recognized as such and converted to Perl's internal representation only if the appropri-ate appropriate
ate encoding is specified.
Unicode characters can also be added to a string by using the "\x{...}" notation. The Unicode
code for the desired character, in hexadecimal, should be placed in the braces. For instance, a
smiley face is "\x{263A}". This encoding scheme only works for characters with a code of 0x100
or above.
Additionally, if you
use charnames ':full';
you can use the "\N{...}" notation and put the official Unicode character name within the braces,
such as "\N{WHITE SMILING FACE}".
If an appropriate encoding is specified, identifiers within the Perl script may contain Unicode
alphanumeric characters, including ideographs. Perl does not currently attempt to canonicalize
variable names.
Regular expressions match characters instead of bytes. "." matches a character instead of a
byte. The "\C" pattern is provided to force a match a single byte--a "char" in C, hence "\C".
Character classes in regular expressions match characters instead of bytes and match against the
character properties specified in the Unicode properties database. "\w" can be used to match a
Japanese ideograph, for instance.
(However, and as a limitation of the current implementation, using "\w" or "\W" inside a "[...]"
character class will still match with byte semantics.)
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 conve-nience convenience
nience you can use dashes, spaces, or underbars, and case is unimportant. It is recommended, how-ever, however,
ever, 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 3.2.0, March 2002, or
Perl 5.8.0, July 2002. Unicode 4.0.0 came out in April 2003, and Perl 5.8.1 in September 2003.
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.
Because scripts differ in their directionality--Hebrew is written right to left, for exam-ple--Unicode example--Unicode
ple--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{Cyril-lic}", "\p{Cyrillic}",
lic}", are as follows:
Arabic
Armenian
Bengali
Bopomofo
Buhid
CanadianAboriginal
Cherokee
Cyrillic
Deseret
Devanagari
Ethiopic
Georgian
Gothic
Greek
Gujarati
Gurmukhi
Han
Hangul
Hanunoo
Hebrew
Hiragana
Inherited
Kannada
Katakana
Khmer
Lao
Latin
Malayalam
Mongolian
Myanmar
Ogham
OldItalic
Oriya
Runic
Sinhala
Syriac
Tagalog
Tagbanwa
Tamil
Telugu
Thaana
Thai
Tibetan
Yi
Extended property classes can supplement the basic properties, defined by the PropList Unicode data-base: database:
base:
ASCIIHexDigit
BidiControl
Dash
Deprecated
Diacritic
Extender
GraphemeLink
HexDigit
Hyphen
Ideographic
IDSBinaryOperator
IDSTrinaryOperator
JoinControl
LogicalOrderException
NoncharacterCodePoint
OtherAlphabetic
OtherDefaultIgnorableCodePoint
OtherGraphemeExtend
OtherLowercase
OtherMath
OtherUppercase
QuotationMark
Radical
SoftDotted
TerminalPunctuation
UnifiedIdeograph
WhiteSpace
and there are further derived properties:
Alphabetic Lu + Ll + Lt + Lm + Lo + OtherAlphabetic
Lowercase Ll + OtherLowercase
Uppercase Lu + OtherUppercase
Math Sm + OtherMath
ID_Start Lu + Ll + Lt + Lm + Lo + Nl
ID_Continue ID_Start + Mn + Mc + Nd + Pc
Any Any character
Assigned Any non-Cn character (i.e. synonym for \P{Cn})
Unassigned Synonym for \p{Cn}
Common Any character (or unassigned code point)
not explicitly assigned to a script
For backward compatibility (with Perl 5.6), all properties mentioned so far may have "Is" prepended
to their name, so "\P{IsLu}", for example, is equal to "\P{Lu}".
Blocks
In addition to scripts, Unicode also defines blocks of characters. The difference between scripts
and blocks is that the concept of scripts is closer to natural languages, while the concept of blocks
is more of an artificial grouping based on groups of 256 Unicode characters. For example, the "Latin"
script contains letters from many blocks but does not contain all the characters from those blocks.
It does not, for example, contain digits, because digits are shared across many scripts. Digits and
similar groups, like punctuation, are in a category called "Common".
For more about scripts, see the UTR #24:
http://www.unicode.org/unicode/reports/tr24/
For more about blocks, see:
http://www.unicode.org/Public/UNIDATA/Blocks.txt
Block names are given with the "In" prefix. For example, the Katakana block is referenced via
"\p{InKatakana}". The "In" prefix may be omitted if there is no naming conflict with a script or any
other property, but it is recommended that "In" always be used for block tests to avoid confusion.
These block names are supported:
InAlphabeticPresentationForms
InArabic
InArabicPresentationFormsA
InArabicPresentationFormsB
InArmenian
InArrows
InBasicLatin
InBengali
InBlockElements
InBopomofo
InBopomofoExtended
InBoxDrawing
InBraillePatterns
InBuhid
InByzantineMusicalSymbols
InCJKCompatibility
InCJKCompatibilityForms
InCJKCompatibilityIdeographs
InCJKCompatibilityIdeographsSupplement
InCJKRadicalsSupplement
InCJKSymbolsAndPunctuation
InCJKUnifiedIdeographs
InCJKUnifiedIdeographsExtensionA
InCJKUnifiedIdeographsExtensionB
InCherokee
InCombiningDiacriticalMarks
InCombiningDiacriticalMarksforSymbols
InCombiningHalfMarks
InControlPictures
InCurrencySymbols
InCyrillic
InCyrillicSupplementary
InDeseret
InDevanagari
InDingbats
InEnclosedAlphanumerics
InEnclosedCJKLettersAndMonths
InEthiopic
InGeneralPunctuation
InGeometricShapes
InGeorgian
InGothic
InGreekExtended
InGreekAndCoptic
InGujarati
InGurmukhi
InHalfwidthAndFullwidthForms
InHangulCompatibilityJamo
InHangulJamo
InHangulSyllables
InHanunoo
InHebrew
InHighPrivateUseSurrogates
InHighSurrogates
InHiragana
InIPAExtensions
InIdeographicDescriptionCharacters
InKanbun
InKangxiRadicals
InKannada
InKatakana
InKatakanaPhoneticExtensions
InKhmer
InLao
InLatin1Supplement
InLatinExtendedA
InLatinExtendedAdditional
InLatinExtendedB
InLetterlikeSymbols
InLowSurrogates
InMalayalam
InMathematicalAlphanumericSymbols
InMathematicalOperators
InMiscellaneousMathematicalSymbolsA
InMiscellaneousMathematicalSymbolsB
InMiscellaneousSymbols
InMiscellaneousTechnical
InMongolian
InMusicalSymbols
InMyanmar
InNumberForms
InOgham
InOldItalic
InOpticalCharacterRecognition
InOriya
InPrivateUseArea
InRunic
InSinhala
InSmallFormVariants
InSpacingModifierLetters
InSpecials
InSuperscriptsAndSubscripts
InSupplementalArrowsA
InSupplementalArrowsB
InSupplementalMathematicalOperators
InSupplementaryPrivateUseAreaA
InSupplementaryPrivateUseAreaB
InSyriac
InTagalog
InTagbanwa
InTags
InTamil
InTelugu
InThaana
InThai
InTibetan
InUnifiedCanadianAboriginalSyllabics
InVariationSelectors
InYiRadicals
InYiSyllables
The special pattern "\X" matches any extended Unicode sequence--"a combining character sequence"
in Standardese--where the first character is a base character and subsequent characters are mark
characters that apply to the base character. "\X" is equivalent to "(?:\PM\pM*)".
The "tr///" operator translates characters instead of bytes. Note that the "tr///CU" functional-
ity has been removed. For similar functionality see pack('U0', ...) and pack('C0', ...).
Case translation operators use the Unicode case translation tables when character input is pro-
vided. Note that "uc()", or "\U" in interpolated strings, translates to uppercase, while
"ucfirst", or "\u" in interpolated strings, translates to titlecase in languages that make the
distinction.
Most operators that deal with positions or lengths in a string will automatically switch to using
character positions, including "chop()", "chomp()", "substr()", "pos()", "index()", "rindex()",
"sprintf()", "write()", and "length()". Operators that specifically do not switch include
"vec()", "pack()", and "unpack()". Operators that really don't care include operators that
treats strings as a bucket of bits such as "sort()", and operators dealing with filenames.
The "pack()"/"unpack()" letters "c" and "C" do not change, since they are often used for byte-
oriented formats. Again, think "char" in the C language.
There is a new "U" specifier that converts between Unicode characters and code points.
The "chr()" and "ord()" functions work on characters, similar to "pack("U")" and "unpack("U")",
not "pack("C")" and "unpack("C")". "pack("C")" and "unpack("C")" are methods for emulating byte-
oriented "chr()" and "ord()" on Unicode strings. While these methods reveal the internal encod-
ing of Unicode strings, that is not something one normally needs to care about at all.
The bit string operators, "& | ^ ~", can operate on character data. However, for backward com-
patibility, such as when using bit string operations when characters are all less than 256 in
ordinal value, one should not use "~" (the bit complement) with characters of both values less
than 256 and values greater than 256. Most importantly, DeMorgan's laws ("~($x|$y) eq ~$x&~$y"
and "~($x&$y) eq ~$x|~$y") will not hold. The reason for this mathematical faux pas is that the
complement cannot return both the 8-bit (byte-wide) bit complement and the full character-wide
bit complement.
lc(), uc(), lcfirst(), and ucfirst() work for the following cases:
the case mapping is from a single Unicode character to another single Unicode character,
or
the case mapping is from a single Unicode character to more than one Unicode character.
Things to do with locales (Lithuanian, Turkish, Azeri) do not work since Perl does not understand
the concept of Unicode locales.
See the Unicode Technical Report #21, Case Mappings, for more details.
And finally, "scalar reverse()" reverses by character rather than by byte.
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:
Two hexadecimal numbers separated by horizontal whitespace (space or tabular characters) denoting
a range of Unicode code points to include.
Something to include, prefixed by "+": a built-in character property (prefixed by "utf8::") or a
user-defined character property, to represent all the characters in that property; two hexadeci-
mal code points for a range; or a single hexadecimal code point.
Something to exclude, prefixed by "-": an existing character property (prefixed by "utf8::") or a
user-defined character property, to represent all the characters in that property; two hexadeci-
mal code points for a range; or a single hexadecimal code point.
Something to negate, prefixed "!": an existing character property (prefixed by "utf8::") or a
user-defined character property, to represent all the characters in that property; two hexadeci-
mal code points for a range; or a single hexadecimal code point.
Something to intersect with, prefixed by "&": an existing character property (prefixed by
"utf8::") or a user-defined character property, for all the characters except the characters in
the property; two hexadecimal code points for a range; or a single hexadecimal code point.
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 noth-
ing (resulting in an empty set).
You can also define your own mappings to be used in the lc(), lcfirst(), uc(), and ucfirst() (or
their string-inlined versions). The principle is the same: define subroutines in the "main" package
with names like "ToLower" (for lc() and lcfirst()), "ToTitle" (for the first character in ucfirst()),
and "ToUpper" (for uc(), and the rest of the characters in ucfirst()).
The string returned by the subroutines needs now to be three hexadecimal numbers separated by tabula-
tors: 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/Special-
Casing.txt. The "Digit" and "Fold" mappings that one can see in the directory are not directly
user-accessible, one can use either the "Unicode::UCD" module, or just match case-insensitively
(that's when the "Fold" mapping is used).
A final note on the user-defined property tests and mappings: they will be used only if the scalar
has been marked as having Unicode characters. Old byte-style strings will not be affected.
Character Encodings for Input and Output
See Encode.
Unicode Regular Expression Support Level
The following list of Unicode support for regular expressions describes all the features currently
supported. The references to "Level N" and the section numbers refer to the Unicode Technical Report
18, "Unicode Regular Expression Guidelines", version 6 (Unicode 3.2.0, Perl 5.8.0).
Level 1 - Basic Unicode Support
2.1 Hex Notation - done [1]
Named Notation - done [2]
2.2 Categories - done [3][4]
2.3 Subtraction - MISSING [5][6]
2.4 Simple Word Boundaries - done [7]
2.5 Simple Loose Matches - done [8]
2.6 End of Line - MISSING [9][10]
[ 1] \x{...}
[ 2] \N{...}
[ 3] . \p{...} \P{...}
[ 4] support for scripts (see UTR#24 Script Names), blocks,
binary properties, enumerated non-binary properties, and
numeric properties (as listed in UTR#18 Other Properties)
[ 5] have negation
[ 6] can use regular expression look-ahead [a]
or user-defined character properties [b] to emulate subtraction
[ 7] include Letters in word characters
[ 8] note that Perl does Full case-folding in matching, not Simple:
for example U+1F88 is equivalent with U+1F00 U+03B9,
not with 1F80. This difference matters for certain Greek
capital letters with certain modifiers: the Full case-folding
decomposes the letter, while the Simple case-folding would map
it to a single character.
[ 9] see UTR #13 Unicode Newline Guidelines
[10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029}
(should also affect <>, $., and script line numbers)
(the \x{85}, \x{2028} and \x{2029} do match \s)
[a] You can mimic class subtraction using lookahead. For example, what UTR #18 might write as
[{Greek}-[{UNASSIGNED}]]
in Perl can be written as:
(?!\p{Unassigned})\p{InGreekAndCoptic}
(?=\p{Assigned})\p{InGreekAndCoptic}
But in this particular example, you probably really want
\p{GreekAndCoptic}
which will match assigned characters known to be part of the Greek script.
Also see the Unicode::Regex::Set module, it does implement the full UTR #18 grouping, intersec-
tion, union, and removal (subtraction) syntax.
[b] See "User-Defined Character Properties".
Level 2 - Extended Unicode Support
3.1 Surrogates - MISSING [11]
3.2 Canonical Equivalents - MISSING [12][13]
3.3 Locale-Independent Graphemes - MISSING [14]
3.4 Locale-Independent Words - MISSING [15]
3.5 Locale-Independent Loose Matches - MISSING [16]
[11] Surrogates are solely a UTF-16 concept and Perl's internal
representation is UTF-8. The Encode module does UTF-16, though.
[12] see UTR#15 Unicode Normalization
[13] have Unicode::Normalize but not integrated to regexes
[14] have \X but at this level . should equal that
[15] need three classes, not just \w and \W
[16] see UTR#21 Case Mappings
Level 3 - Locale-Sensitive Support
4.1 Locale-Dependent Categories - MISSING
4.2 Locale-Dependent Graphemes - MISSING [16][17]
4.3 Locale-Dependent Words - MISSING
4.4 Locale-Dependent Loose Matches - MISSING
4.5 Locale-Dependent Ranges - MISSING
[16] see UTR#10 Unicode Collation Algorithms
[17] have Unicode::Collate but not integrated to regexes
Unicode Encodings
Unicode characters are assigned to code points, which are abstract numbers. To use these numbers,
various encodings are needed.
UTF-8
UTF-8 is a variable-length (1 to 6 bytes, current character allocations require 4 bytes), byte-
order independent encoding. For ASCII (and we really do mean 7-bit ASCII, not another 8-bit
encoding), UTF-8 is transparent.
The following table is from Unicode 3.2.
Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
U+0000..U+007F 00..7F
U+0080..U+07FF C2..DF 80..BF
U+0800..U+0FFF E0 A0..BF 80..BF
U+1000..U+CFFF E1..EC 80..BF 80..BF
U+D000..U+D7FF ED 80..9F 80..BF
U+D800..U+DFFF ******* ill-formed *******
U+E000..U+FFFF EE..EF 80..BF 80..BF
U+10000..U+3FFFF F0 90..BF 80..BF 80..BF
U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
U+100000..U+10FFFF F4 80..8F 80..BF 80..BF
Note the "A0..BF" in "U+0800..U+0FFF", the "80..9F" in "U+D000...U+D7FF", the "90..B"F in
"U+10000..U+3FFFF", and the "80...8F" in "U+100000..U+10FFFF". The "gaps" are caused by legal
UTF-8 avoiding non-shortest encodings: it is technically possible to UTF-8-encode a single code
point in different ways, but that is explicitly forbidden, and the shortest possible encoding
should always be used. So that's what Perl does.
Another way to look at it is via bits:
Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
0aaaaaaa 0aaaaaaa
00000bbbbbaaaaaa 110bbbbb 10aaaaaa
ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
00000dddccccccbbbbbbaaaaaa 11110ddd 10cccccc 10bbbbbb 10aaaaaa
As you can see, the continuation bytes all begin with 10, and the leading bits of the start byte
tell how many bytes the are in the encoded character.
UTF-EBCDIC
Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
UTF-16, UTF-16BE, UTF-16LE, Surrogates, and BOMs (Byte Order Marks)
The followings items are mostly for reference and general Unicode knowledge, Perl doesn't use
these constructs internally.
UTF-16 is a 2 or 4 byte encoding. The Unicode code points "U+0000..U+FFFF" are stored in a sin-
gle 16-bit unit, and the code points "U+10000..U+10FFFF" in two 16-bit units. The latter case is
using surrogates, the first 16-bit unit being the high surrogate, and the second being the low
surrogate.
Surrogates are code points set aside to encode the "U+10000..U+10FFFF" range of Unicode code
points in pairs of 16-bit units. The high surrogates are the range "U+D800..U+DBFF", and the low
surrogates are the range "U+DC00..U+DFFF". The surrogate encoding is
$hi = ($uni - 0x10000) / 0x400 + 0xD800;
$lo = ($uni - 0x10000) % 0x400 + 0xDC00;
and the decoding is
$uni = 0x10000 + ($hi - 0xD800) * 0x400 + ($lo - 0xDC00);
If you try to generate surrogates (for example by using chr()), you will get a warning if warn-
ings are turned on, because those code points are not valid for a Unicode character.
Because of the 16-bitness, UTF-16 is byte-order dependent. UTF-16 itself can be used for in-mem-
ory computations, but if storage or transfer is required either UTF-16BE (big-endian) or UTF-16LE
(little-endian) encodings must be chosen.
This introduces another problem: what if you just know that your data is UTF-16, but you don't
know which endianness? Byte Order Marks, or BOMs, are a solution to this. A special character
has been reserved in Unicode to function as a byte order marker: the character with the code
point "U+FEFF" is the BOM.
The trick is that if you read a BOM, you will know the byte order, since if it was written on a
big-endian platform, you will read the bytes "0xFE 0xFF", but if it was written on a little-
endian platform, you will read the bytes "0xFF 0xFE". (And if the originating platform was writ-
ing in UTF-8, you will read the bytes "0xEF 0xBB 0xBF".)
The way this trick works is that the character with the code point "U+FFFE" is guaranteed not to
be a valid Unicode character, so the sequence of bytes "0xFF 0xFE" is unambiguously "BOM, repre-
sented in little-endian format" and cannot be "U+FFFE", represented in big-endian format".
UTF-32, UTF-32BE, UTF-32LE
The UTF-32 family is pretty much like the UTF-16 family, expect that the units are 32-bit, and
therefore the surrogate scheme is not needed. The BOM signatures will be "0x00 0x00 0xFE 0xFF"
for BE and "0xFF 0xFE 0x00 0x00" for LE.
UCS-2, UCS-4
Encodings defined by the ISO 10646 standard. UCS-2 is a 16-bit encoding. Unlike UTF-16, UCS-2
is not extensible beyond "U+FFFF", because it does not use surrogates. UCS-4 is a 32-bit encod-
ing, functionally identical to UTF-32.
UTF-7
A seven-bit safe (non-eight-bit) encoding, which is useful if the transport or storage is not
eight-bit safe. Defined by RFC 2152.
Security Implications of Unicode
Malformed UTF-8
Unfortunately, the specification of UTF-8 leaves some room for interpretation of how many bytes
of encoded output one should generate from one input Unicode character. Strictly speaking, the
shortest possible sequence of UTF-8 bytes should be generated, because otherwise there is poten-
tial for an input buffer overflow at the receiving end of a UTF-8 connection. Perl always gener-
ates the shortest length UTF-8, and with warnings on Perl will warn about non-shortest length
UTF-8 along with other malformations, such as the surrogates, which are not real Unicode code
points.
Regular expressions behave slightly differently between byte data and character (Unicode) data.
For example, the "word character" character class "\w" will work differently depending on if data
is eight-bit bytes or Unicode.
In the first case, the set of "\w" characters is either small--the default set of alphabetic
characters, digits, and the "_"--or, if you are using a locale (see perllocale), the "\w" might
contain a few more letters according to your language and country.
In the second case, the "\w" set of characters is much, much larger. Most importantly, even in
the set of the first 256 characters, it will probably match different characters: unlike most
locales, which are specific to a language and country pair, Unicode classifies all the characters
that are letters somewhere as "\w". For example, your locale might not think that LATIN SMALL
LETTER ETH is a letter (unless you happen to speak Icelandic), but Unicode does.
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 neces-
sary. If your legacy code does not explicitly use Unicode, no automatic switch-over to charac-
ters 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. 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 plat-
form'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:
You can enable automatic UTF-8-ification of your standard file handles, default "open()" layer,
and @ARGV by using either the "-C" command line switch or the "PERL_UNICODE" environment vari-
able, see perlrun for the documentation of the "-C" switch.
Perl tries really hard to work both with Unicode and the old byte-oriented world. Most often this
is nice, but sometimes Perl's straddling of the proverbial fence causes problems.
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. Simi-
larly for the qx and system: how well will the 'command line interface' (and which of them?) handle
Unicode?
chdir, chmod, chown, chroot, exec, link, lstat, mkdir, rename, rmdir, stat, symlink, truncate,
unlink, utime, -X
%ENV
glob (aka the <*>)
open, opendir, sysopen
qx (aka the backtick operator), system
readdir, readlink
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.
"DO_UTF8(sv)" returns true if the "UTF8" flag is on and the bytes pragma is not in effect.
"SvUTF8(sv)" returns true is the "UTF8" flag is on; the bytes pragma is ignored. The "UTF8" flag
being on does not mean that there are any characters of code points greater than 255 (or 127) in
the scalar or that there are even any characters in the scalar. What the "UTF8" flag means is
that the sequence of octets in the representation of the scalar is the sequence of UTF-8 encoded
code points of the characters of a string. The "UTF8" flag being off means that each octet in
this representation encodes a single character with code point 0..255 within the string. Perl's
Unicode model is not to use UTF-8 until it is absolutely necessary.
"uvuni_to_utf8(buf, chr)" writes a Unicode character code point into a buffer encoding the code
point as UTF-8, and returns a pointer pointing after the UTF-8 bytes.
"utf8_to_uvuni(buf, lenp)" reads UTF-8 encoded bytes from a buffer and returns the Unicode char-
acter code point and, optionally, the length of the UTF-8 byte sequence.
"utf8_length(start, end)" returns the length of the UTF-8 encoded buffer in characters.
"sv_len_utf8(sv)" returns the length of the UTF-8 encoded scalar.
"sv_utf8_upgrade(sv)" converts the string of the scalar to its UTF-8 encoded form.
"sv_utf8_downgrade(sv)" does the opposite, if possible. "sv_utf8_encode(sv)" is like
sv_utf8_upgrade except that it does not set the "UTF8" flag. "sv_utf8_decode()" does the oppo-
site of "sv_utf8_encode()". Note that none of these are to be used as general-purpose encoding
or decoding interfaces: "use Encode" for that. "sv_utf8_upgrade()" is affected by the encoding
pragma but "sv_utf8_downgrade()" is not (since the encoding pragma is designed to be a one-way
street).
is_utf8_char(s) returns true if the pointer points to a valid UTF-8 character.
"is_utf8_string(buf, len)" returns true if "len" bytes of the buffer are valid UTF-8.
"UTF8SKIP(buf)" will return the number of bytes in the UTF-8 encoded character in the buffer.
"UNISKIP(chr)" will return the number of bytes required to UTF-8-encode the Unicode character
code point. "UTF8SKIP()" is useful for example for iterating over the characters of a UTF-8
encoded buffer; "UNISKIP()" is useful, for example, in computing the size required for a UTF-8
encoded buffer.
"utf8_distance(a, b)" will tell the distance in characters between the two pointers pointing to
the same UTF-8 encoded buffer.
"utf8_hop(s, off)" will return a pointer to an UTF-8 encoded buffer that is "off" (positive or
negative) Unicode characters displaced from the UTF-8 buffer "s". Be careful not to overstep the
buffer: "utf8_hop()" will merrily run off the end or the beginning of the buffer if told to do
so.
"pv_uni_display(dsv, spv, len, pvlim, flags)" and "sv_uni_display(dsv, ssv, pvlim, flags)" are
useful for debugging the output of Unicode strings and scalars. By default they are useful only
for debugging--they display all characters as hexadecimal code points--but with the flags
"UNI_DISPLAY_ISPRINT", "UNI_DISPLAY_BACKSLASH", and "UNI_DISPLAY_QQ" you can make the output more
readable.
"ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2)" can be used to compare two strings case-insen-
sitively in Unicode. For case-sensitive comparisons you can just use "memEQ()" and "memNE()" as
usual.
For more information, see perlapi, and utf8.c and utf8.h in the Perl source code distribution.
BUGS
Interaction with Locales
Use of locales with Unicode data may lead to odd results. Currently, Perl attempts to attach 8-bit
locale info to characters in the range 0..255, but this technique is demonstrably incorrect for
locales that use characters above that range when mapped into Unicode. Perl's Unicode support will
also tend to run slower. Use of locales with Unicode is discouraged.
Interaction with Extensions
When Perl exchanges data with an extension, the extension should be able to understand the UTF-8 flag
and act accordingly. If the extension doesn't know about the flag, it's likely that the extension
will return incorrectly-flagged data.
So if you're working with Unicode data, consult the documentation of every module you're using if
there are any issues with Unicode data exchange. If the documentation does not talk about Unicode at
all, suspect the worst and probably look at the source to learn how the module is implemented. Mod-
ules 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 $ret;
}
}
Some extensions provide filters on data entry/exit points, such as DB_File::filter_store_key and fam-
ily. Look out for such filters in the documentation of your extensions, they can make the transition
to Unicode data much easier.
Speed
Some functions are slower when working on UTF-8 encoded strings than on byte encoded strings. All
functions that need to hop over characters such as length(), substr() or index(), or matching regular
expressions can work much faster when the underlying data are byte-encoded.
In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a caching scheme was introduced
which will hopefully make the slowness somewhat less spectacular, at least for some operations. In
general, operations with UTF-8 encoded strings are still slower. As an example, the Unicode proper-
ties (character classes) like "\p{Nd}" are known to be quite a bit slower (5-20 times) than their
simpler counterparts like "\d" (then again, there 268 Unicode characters matching "Nd" compared with
the 10 ASCII characters matching "d").
Porting code from perl-5.6.X
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer was required to use the "utf8"
pragma to declare that a given scope expected to deal with Unicode data and had to make sure that
only Unicode data were reaching that scope. If you have code that is working with 5.6, you will need
some of the following adjustments to your code. The examples are written such that the code will con-
tinue to work under 5.6, so you should be safe to try them out.
A filehandle that should read or write UTF-8
if ($] > 5.007) {
binmode $fh, ":utf8";
}
A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension that has no mention of Unicode in the man-
page, you need to make sure that the UTF-8 flag is stripped off. Note that at the time of this
writing (October 2002) the mentioned modules are not UTF-8-aware. Please check the documentation
to verify if this is still true.
if ($] > 5.007) {
require Encode;
$val = Encode::encode_utf8($val); # make octets
}
A scalar we got back from an extension
If you believe the scalar comes back as UTF-8, you will most likely want the UTF-8 flag restored:
if ($] > 5.007) {
require Encode;
$val = Encode::decode_utf8($val);
}
Same thing, if you are really sure it is UTF-8
if ($] > 5.007) {
require Encode;
Encode::_utf8_on($val);
}
A wrapper for fetchrow_array and fetchrow_hashref
When the database contains only UTF-8, a wrapper function or method is a convenient way to
replace all your fetchrow_array and fetchrow_hashref calls. A wrapper function will also make it
easier to adapt to future enhancements in your database driver. Note that at the time of this
writing (October 2002), the DBI has no standardized way to deal with UTF-8 data. Please check the
documentation to verify if that is still true.
sub fetchrow {
my($self, $sth, $what) = @_; # $what is one of fetchrow_{array,hashref}
if ($] < 5.007) {
return $sth->$what;
} else {
require Encode;
if (wantarray) {
my @arr = $sth->$what;
for (@arr) {
defined && /[^\000-\177]/ && Encode::_utf8_on($_);
}
return @arr;
} else {
my $ret = $sth->$what;
if (ref $ret) {
for my $k (keys %$ret) {
defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret->{$k};
}
return $ret;
} else {
defined && /[^\000-\177]/ && Encode::_utf8_on($_) for $ret;
return $ret;
}
}
}
}
A large scalar that you know can only contain ASCII
Scalars that contain only ASCII and are marked as UTF-8 are sometimes a drag to your program. If
you recognize such a situation, just remove the UTF-8 flag:
utf8::downgrade($val) if $] > 5.007;
SEE ALSO
perluniintro, encoding, Encode, open, utf8, bytes, perlretut, "${^UNICODE}" in perlvar
perl v5.8.8 2006-01-07 PERLUNICODE(1)
|