PERLUNICODE(1) Perl Programmers Reference Guide PERLUNICODE(1)
perlunicode - Unicode support in Perl
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 ":encod-
ing(...)" layer. See open.
To indicate that Perl source itself is using a particu-
lar 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 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 expli-
cit "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 encod-
ing.
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
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unicode model: implicit upgrading from byte strings to
Unicode strings assumes that they were encoded in ISO
8859-1 (Latin-1), but Unicode strings are downgraded
with UTF-8 encoding. This happens because the first 256
codepoints in Unicode happens to agree with Latin-1.
If you wish to interpret byte strings as UTF-8 instead,
use the "encoding" pragma:
use encoding 'utf8';
See "Byte and Character Semantics" for more details.
Byte and Character Semantics
Beginning with version 5.6, Perl uses logically-wide charac-
ters 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 charac-
ter 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 infor-
mation from the user, Perl decides in favor of compatibility
and chooses to use byte semantics.
This behavior preserves compatibility with earlier versions
of Perl, which allowed byte semantics in Perl operations
only if none of the program's inputs were marked as being as
source of Unicode character data. Such data may come from
filehandles, from calls to external programs, from informa-
tion 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 encoun-
tered by the parser. Note that this pragma is only required
while Perl defaults to byte semantics; when character seman-
tics 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
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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 appropriate 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}".
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+ 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 charac-
ters 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 implementa-
tion, 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 convenience you can
use dashes, spaces, or underbars, and case is unimpor-
tant. It is recommended, however, that for consistency
you use the following naming: the official Unicode
script, property, or block name (see below for the addi-
tional 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.
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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
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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 example--Unicode supplies
these properties in the BidiClass class:
Property Meaning
L Left-to-Right
LRE Left-to-Right Embedding
LRO Left-to-Right Override
R Right-to-Left
AL Right-to-Left Arabic
RLE Right-to-Left Embedding
RLO Right-to-Left Override
PDF Pop Directional Format
EN European Number
ES European Number Separator
ET European Number Terminator
AN Arabic Number
CS Common Number Separator
NSM Non-Spacing Mark
BN Boundary Neutral
B Paragraph Separator
S Segment Separator
WS Whitespace
ON Other Neutrals
For example, "\p{BidiClass:R}" matches characters that
are normally written right to left.
Scripts
The script names which can be used by "\p{...}" and
"\P{...}", such as in "\p{Latin}" or "\p{Cyrillic}", are as
follows:
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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 proper-
ties, defined by the PropList Unicode database:
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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 char-
acters. The difference between scripts and blocks is that
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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 punc-
tuation, 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:
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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
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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
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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 charac-
ter 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" functionality has been
removed. For similar functionality see pack('U0', ...)
and pack('C0', ...).
+ Case translation operators use the Unicode case transla-
tion tables when character input is provided. 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 for-
mats. 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 encoding of Unicode strings,
that is not something one normally needs to care about
at all.
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+ The bit string operators, "& | ^ ~", can operate on
character data. However, for backward compatibility,
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 fol-
lowing cases:
+ the case mapping is from a single Unicode char-
acter to another single Unicode character, or
+ the case mapping is from a single Unicode char-
acter 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 sub-
routines 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.
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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 whi-
tespace (space or tabular characters) denoting a range
of Unicode code points to include.
+ Something to include, prefixed by "+": a built-in char-
acter property (prefixed by "utf8::") or a user-defined
character property, to represent all the characters in
that property; two hexadecimal code points for a range;
or a single hexadecimal code point.
+ Something to exclude, prefixed by "-": an existing char-
acter property (prefixed by "utf8::") or a user-defined
character property, to represent all the characters in
that property; two hexadecimal 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 char-
acter property, to represent all the characters in that
property; two hexadecimal code points for a range; or a
single hexadecimal code point.
+ Something to intersect with, prefixed by "&": an exist-
ing 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:
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sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
END
}
Suppose you wanted to match only the allocated characters,
not the raw block ranges: in other words, you want to remove
the non-characters:
sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
-utf8::IsCn
END
}
The negation is useful for defining (surprise!) negated
classes.
sub InNotKana {
return <<'END';
!utf8::InHiragana
-utf8::InKatakana
+utf8::IsCn
END
}
Intersection is useful for getting the common characters
matched by two (or more) classes.
sub InFooAndBar {
return <<'END';
+main::Foo
&main::Bar
END
}
It's important to remember not to use "&" for the first set
-- that would be intersecting with nothing (resulting in an
empty set).
You can also define your own mappings to be used in the
lc(), lcfirst(), uc(), and ucfirst() (or their string-
inlined versions). The principle is the same: define subrou-
tines 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()).
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The string returned by the subroutines needs now to be three
hexadecimal numbers separated by tabulators: start of the
source range, end of the source range, and start of the des-
tination 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 char-
acters will remain unchanged.
If there is no source range to speak of, that is, the map-
ping is from a single character to another single character,
leave the end of the source range empty, but the two tabula-
tor characters are still needed. For example:
sub ToLower {
return <<END;
0041\t\t0061
END
}
defines a lc() mapping that causes only "A" to be mapped to
"a", all other characters will remain unchanged.
(For serious hackers only) If you want to introspect the
default mappings, you can find the data in the directory
$Config{privlib}/unicore/To/. The mapping data is returned
as the here-document, and the "utf8::ToSpecFoo" are special
exception mappings derived from
<$Config{privlib}>/unicore/SpecialCasing.txt. The "Digit"
and "Fold" mappings that one can see in the directory are
not directly user-accessible, one can use either the
"Unicode::UCD" module, or just match case-insensitively
(that's when the "Fold" mapping is used).
A final note on the user-defined property tests and map-
pings: 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.
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Unicode Regular Expression Support Level
The following list of Unicode support for regular expres-
sions 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}
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which will match assigned characters known to be part of
the Greek script.
Also see the Unicode::Regex::Set module, it does imple-
ment the full UTR #18 grouping, intersection, 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 char-
acter allocations require 4 bytes), byte-order indepen-
dent encoding. For ASCII (and we really do mean 7-bit
ASCII, not another 8-bit encoding), UTF-8 is tran-
sparent.
The following table is from Unicode 3.2.
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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 encod-
ings: it is technically possible to UTF-8-encode a sin-
gle 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 gen-
eral Unicode knowledge, Perl doesn't use these con-
structs internally.
UTF-16 is a 2 or 4 byte encoding. The Unicode code
points "U+0000..U+FFFF" are stored in a single 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
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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 warnings are turned
on, because those code points are not valid for a
Unicode character.
Because of the 16-bitness, UTF-16 is byte-order depen-
dent. UTF-16 itself can be used for in-memory computa-
tions, but if storage or transfer is required either
UTF-16BE (big-endian) or UTF-16LE (little-endian) encod-
ings must be chosen.
This introduces another problem: what if you just know
that your data is UTF-16, but you don't know which endi-
anness? 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 writing 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, represented in little-endian for-
mat" 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
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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 encoding, functionally identical to
UTF-32.
+ UTF-7
A seven-bit safe (non-eight-bit) encoding, which is use-
ful 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 char-
acter. Strictly speaking, the shortest possible
sequence of UTF-8 bytes should be generated, because
otherwise there is potential for an input buffer over-
flow at the receiving end of a UTF-8 connection. Perl
always generates 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 surro-
gates, 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 dif-
ferently 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 perllo-
cale), the "\w" might contain a few more letters accord-
ing 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
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example, your locale might not think that LATIN SMALL
LETTER ETH is a letter (unless you happen to speak Ice-
landic), 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 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 acciden-
tally mix bytes and characters, however (see perluniin-
tro), 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 encod-
ing in this document and elsewhere should be read as meaning
the UTF-EBCDIC specified in Unicode Technical Report 16,
unless ASCII vs. EBCDIC issues are specifically discussed.
There is no "utfebcdic" pragma or ":utfebcdic" layer;
rather, "utf8" and ":utf8" are reused to mean the platform's
"natural" 8-bit encoding of Unicode. See perlebcdic for more
discussion of the issues.
Locales
Usually locale settings and Unicode do not affect each
other, but there are a couple of exceptions:
+ You can enable automatic UTF-8-ification of your stan-
dard file handles, default "open()" layer, and @ARGV by
using either the "-C" command line switch or the
"PERL_UNICODE" environment variable, 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.
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The following are such interfaces. For all of these inter-
faces 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 depen-
dent on the operating system and the filesystem(s). For
example, whether filenames can be in Unicode, and in exactly
what kind of encoding, is not exactly a portable concept.
Similarly for the qx and system: how well will the 'command
line interface' (and which of them?) handle Unicode?
+ 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 sud-
denly 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 Sup-
port" 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
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there are any characters of code points greater than 255
(or 127) in the scalar or that there are even any char-
acters 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 character 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 opposite 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 use-
ful for example for iterating over the characters of a
UTF-8 encoded buffer; "UNISKIP()" is useful, for exam-
ple, in computing the size required for a UTF-8 encoded
buffer.
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+ "utf8_distance(a, b)" will tell the distance in charac-
ters 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 read-
able.
+ "ibcmp_utf8(s1, pe1, u1, l1, u1, s2, pe2, l2, u2)" can
be used to compare two strings case-insensitively 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.
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 accord-
ingly. 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 documen-
tation 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.
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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 exten-
sion can handle. Convert arguments passed to the extensions
to that encoding and convert results back from that encod-
ing. Write wrapper functions that do the conversions for
you, so you can later change the functions when the exten-
sion 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 $ret;
}
}
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Some extensions provide filters on data entry/exit points,
such as DB_File::filter_store_key and family. Look out for
such filters in the documentation of your extensions, they
can make the transition to Unicode data much easier.
Speed
Some functions are slower when working on UTF-8 encoded
strings than on byte encoded strings. All functions that
need to hop over characters such as length(), substr() or
index(), or matching regular expressions can work much fas-
ter when the underlying data are byte-encoded.
In Perl 5.8.0 the slowness was often quite spectacular; in
Perl 5.8.1 a caching scheme was introduced which will hope-
fully make the slowness somewhat less spectacular, at least
for some operations. In general, operations with UTF-8
encoded strings are still slower. As an example, the Unicode
properties (character classes) like "\p{Nd}" are known to be
quite a bit slower (5-20 times) than their simpler counter-
parts like "\d" (then again, there 268 Unicode characters
matching "Nd" compared with the 10 ASCII characters matching
"d").
Porting code from perl-5.6.X
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the
programmer was required to use the "utf8" pragma to declare
that a given scope expected to deal with Unicode data and
had to make sure that only Unicode data were reaching that
scope. If you have code that is working with 5.6, you will
need some of the following adjustments to your code. The
examples are written such that the code will continue to
work under 5.6, so you should be safe to try them out.
+ A filehandle that should read or write UTF-8
if ($] > 5.007) {
binmode $fh, ":utf8";
}
+ A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension
that has no mention of Unicode in the manpage, you need
to make sure that the UTF-8 flag is stripped off. Note
that at the time of this writing (October 2002) the men-
tioned modules are not UTF-8-aware. Please check the
documentation to verify if this is still true.
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if ($] > 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 func-
tion 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.
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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 $] > 5.007;
perluniintro, encoding, Encode, open, utf8, bytes, perlre-
tut, "${^UNICODE}" in perlvar
perl v5.8.8 2006-06-30 29