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
":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 recognition of UTF-8
in the Perl scripts themselves (in string or regular
expression literals, or in identifier names) on ASCIIbased
machines or to recognize UTF-EBCDIC on EBCDICbased
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 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 [Toc] [Back]
Beginning with version 5.6, Perl uses logically-wide characters
to represent strings internally.
In future, Perl-level operations will be expected to work
with characters rather than bytes.
However, as an interim compatibility measure, Perl aims to
provide a safe migration path from byte semantics to character
semantics for programs. For operations where Perl
can unambiguously decide that the input data are characters,
Perl switches to character semantics. For operations
where this determination cannot be made without
additional information from the user, Perl decides in
favor of compatibility and chooses to use byte semantics.
This behavior preserves compatibility with earlier versions
of Perl, which allowed byte semantics in Perl operations
only if none of the program's inputs were marked as
being as source of Unicode character data. Such data may
come from filehandles, from calls to external programs,
from information provided by the system (such as %ENV), or
from literals and constants in the source text.
The "bytes" pragma will always, regardless of platform,
force byte semantics in a particular lexical scope. See
bytes.
The "utf8" pragma is primarily a compatibility device that
enables recognition of UTF-(8|EBCDIC) in literals encountered
by the parser. Note that this pragma is only
required while Perl defaults to byte semantics; when character
semantics become the default, this pragma may become
a no-op. See utf8.
Unless explicitly stated, Perl operators use character
semantics for Unicode data and byte semantics for non-Unicode
data. The decision to use character semantics is
made transparently. If input data comes from a Unicode
source--for example, if a character encoding layer is
added to a filehandle or a literal Unicode string constant
appears in a program--character semantics apply. Otherwise,
byte semantics are in effect. The "bytes" pragma
should be used to force byte semantics on Unicode data.
If strings operating under byte semantics and strings with
Unicode character data are concatenated, the new string
will be created by decoding the byte strings as ISO 8859-1
(Latin-1), even if the old Unicode string used EBCDIC.
This translation is done without regard to the system's
native 8-bit encoding. 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 [Toc] [Back]
Character semantics have the following effects:
o 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 ".}" notation. The Unicode code for
the desired character, in hexadecimal, should be
placed in the braces. For instance, a smiley face is
"}". This encoding scheme only works for char
acters with a code of 0x100 or above.
Additionally, if you
use charnames ':full';
you can use the ".}" notation and put the official
Unicode character name within the braces, such as
"HITE SMILING FACE}".
o 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.
o Regular expressions match characters instead of bytes.
"." matches a character instead of a byte. The "
pattern is provided to force a match a single byte--a
"char" in C, hence "
o Character classes in regular expressions match characters
instead of bytes and match against the character
properties specified in the Unicode properties
database. "912
graph, for instance.
(However, and as a limitation of the current implementation,
using "96W" inside a "[...]" character
class will still match with byte semantics.)
o Named Unicode properties, scripts, and block ranges
may be used like character classes via the "{}"
"matches property" construct and the "P{}" negation,
"doesn't match property".
For instance, "{Lu}"
matches any character with the
Unicode "Lu" (Letter, uppercase) property, while
"{M}"
matches any character with an "M"
(mark--accents and such) property. Brackets are not
required for single letter properties, so "{M}"
is
equivalent to "M".
Many predefined properties are
available, such as "{Mirrored}"
and "{Tibetan}".
The official Unicode script and block names have
spaces and dashes as separators, but for convenience
you can use dashes, spaces, or underbars, and case is
unimportant. It is recommended, however, that for consistency
you use the following naming: the official
Unicode script, property, or block name (see below for
the additional rules that apply to block names) with
whitespace and dashes removed, and the words "uppercase-first-lowercase-rest".
"Latin-1 Supplement" thus
becomes "Latin1Supplement".
You can also use negation in both "{}"
and "P{}" by
introducing a caret (^) between the first brace and
the property name: "{^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; "{Lu}"
and "{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 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, "{BidiClass:R}"
matches characters that
are normally written right to left.
Scripts [Toc] [Back]
The script names which can be used by "{...}"
and
"P{...}", such as in "{Latin}"
or "{Cyrillic}",
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 database:
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 {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 [Toc] [Back]
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 "{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
o The special pattern "
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. "
"(?:PMM*)".
o 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', ...).
o Case translation operators use the Unicode case translation
tables when character input is provided. Note
that "uc()", or "U" in interpolated strings, translates
to uppercase, while "ucfirst", or "" in interpolated
strings, translates to titlecase in languages
that make the distinction.
o 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.
o 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.
o 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.
o 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.
o lc(), uc(), lcfirst(), and ucfirst() work for the following
cases:
o the case mapping is from a single Unicode
character to another single Unicode character,
or
o 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.
o And finally, "scalar reverse()" reverses by character
rather than by byte.
User-Defined Character Properties [Toc] [Back]
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 ""
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 ""
or "P" construct.
# assuming property IsForeign defined in Lang::
package main; # property package name required
if ($txt =~ /{Lang::IsForeign}+/)
{ ... }
package Lang; # property package name not required
if ($txt =~ /{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:
o Two hexadecimal numbers separated by horizontal
whitespace (space or tabular characters) denoting a
range of Unicode code points to include.
o Something to include, prefixed by "+": a built-in
character property (prefixed by "utf8::") or a userdefined
character property, to represent all the characters
in that property; two hexadecimal code points
for a range; or a single hexadecimal code point.
o Something to exclude, prefixed by "-": an existing
character property (prefixed by "utf8::") or a userdefined
character property, to represent all the characters
in that property; two hexadecimal code points
for a range; or a single hexadecimal code point.
o 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 hexadecimal code points for a
range; or a single hexadecimal code point.
o 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;
3040309F
30A030FF
END
}
Imagine that the here-doc end marker is at the beginning
of the line. Now you can use "{InKana}"
and
"P{InKana}".
You could also have used the existing block property
names:
sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
END
}
Suppose you wanted to match only the allocated characters,
not the raw block ranges: in other words, you want to
remove the non-characters:
sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
-utf8::IsCn
END
}
The negation is useful for defining (surprise!) negated
classes.
sub InNotKana {
return <<'END';
!utf8::InHiragana
-utf8::InKatakana
+utf8::IsCn
END
}
Intersection is useful for getting the common characters
matched by two (or more) classes.
sub InFooAndBar {
return <<'END';
+main::Foo
&main::Bar
END
}
It's important to remember not to use "&" for the first
set -- that would be intersecting with nothing (resulting
in an empty set).
You can also define your own mappings to be used in the
lc(), lcfirst(), uc(), and ucfirst() (or their stringinlined
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 tabulators: start
of the source range, end of the source range, and start of
the destination range. For example:
sub ToUpper {
return <<END;
006100630041
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;
00410061
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 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 [Toc] [Back]
See Encode.
Unicode Regular Expression Support Level [Toc] [Back]
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).
o 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] .}
[ 2] .}
[ 3] . {...}
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 , and
(should also affect <>, $., and script
line numbers)
(the , and do match
[a] You can mimic class subtraction using lookahead.
For example, what UTR #18 might write as
[{Greek}-[{UNASSIGNED}]]
in Perl can be written as:
(?!{Unassigned}){InGreekAndCoptic}
(?={Assigned}){InGreekAndCoptic}
But in this particular example, you probably really
want
{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, intersection,
union, and removal (subtraction) syntax.
[b] See "User-Defined Character Properties".
o 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 at this level . should equal that
[15] need three classes, not just 72W
[16] see UTR#21 Case Mappings
o 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 [Toc] [Back]
Unicode characters are assigned to code points, which are
abstract numbers. To use these numbers, various encodings
are needed.
o 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.
o UTF-EBCDIC
Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is
ASCII-safe.
o 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 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 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
dependent. UTF-16 itself can be used for in-memory
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 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
format" and cannot be "U+FFFE", represented
in big-endian format".
o 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.
o 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.
o 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 [Toc] [Back]
o 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 potential for an input buffer overflow
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 surrogates,
which are not real Unicode code points.
o Regular expressions behave slightly differently
between byte data and character (Unicode) data. For
example, the "word character" character class "0
will work differently depending on if data is eightbit
bytes or Unicode.
In the first case, the set of "336
either small--the default set of alphabetic characters,
digits, and the "_"--or, if you are using a
locale (see perllocale), the "480
more letters according to your language and country.
In the second case, the "504
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 "1032
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 necessary. If your
legacy code does not explicitly use Unicode, no automatic
switch-over to characters should happen. Characters
shouldn't get downgraded to bytes, either. It
is possible to accidentally mix bytes and characters,
however (see perluniintro), in which case "192
ular expressions might start behaving differently.
Review your code. Use warnings and the "strict"
pragma.
Unicode in Perl on EBCDIC [Toc] [Back]
The way Unicode is handled on EBCDIC platforms is still
experimental. On such platforms, references to UTF-8
encoding in this document and elsewhere should be read as
meaning the UTF-EBCDIC specified in Unicode Technical
Report 16, unless ASCII vs. EBCDIC issues are specifically
discussed. There is no "utfebcdic" pragma or ":utfebcdic"
layer; rather, "utf8" and ":utf8" are reused to mean the
platform's "natural" 8-bit encoding of Unicode. See perlebcdic
for more discussion of the issues.
Locales [Toc] [Back]
Usually locale settings and Unicode do not affect each
other, but there are a couple of exceptions:
o 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 variable, see perlrun for
the documentation of the "-C" switch.
o 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 [Toc] [Back]
While Perl does have extensive ways to input and output in
Unicode, and few other 'entry points' like the @ARGV which
can be interpreted as Unicode (UTF-8), there still are
many places where Unicode (in some encoding or another)
could be given as arguments or received as results, or
both, but it is not.
The following are such interfaces. For all of these
interfaces Perl currently (as of 5.8.3) simply assumes
byte strings both as arguments and results, or UTF-8
strings if the "encoding" pragma has been used.
One reason why Perl does not attempt to resolve the role
of Unicode in this cases is that the answers are highly
dependent on the operating system and the file system(s).
For example, whether filenames can be in Unicode, and in
exactly what kind of encoding, is not exactly a portable
concept. Similarly for the qx and system: how well will
the 'command line interface' (and which of them?) handle
Unicode?
o chmod, chmod, chown, chroot, exec, link, lstat, mkdir,
rename, rmdir, stat, symlink, truncate, unlink, utime,
-X
o %ENV
o glob (aka the <*>)
o open, opendir, sysopen
o qx (aka the backtick operator), system
o readdir, readlink
Forcing Unicode in Perl (Or Unforcing Unicode in Perl) [Toc] [Back]
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 [Toc] [Back]
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.
o "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.
o "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.
o "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.
o "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.
o "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).
o is_utf8_char(s) returns true if the pointer points to
a valid UTF-8 character.
o "is_utf8_string(buf, len)" returns true if "len" bytes
of the buffer are valid UTF-8.
o "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.
o "utf8_distance(a, b)" will tell the distance in characters
between the two pointers pointing to the same
UTF-8 encoded buffer.
o "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.
o "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.
o "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 [Toc] [Back]
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 incorrectlyflagged
data.
So if you're working with Unicode data, consult the documentation
of every module you're using if there are any
issues with Unicode data exchange. If the documentation
does not talk about Unicode at all, suspect the worst and
probably look at the source to learn how the module is
implemented. Modules written completely in Perl shouldn't
cause problems. Modules that directly or indirectly access
code written in other programming languages are at risk.
For affected functions, the simple strategy to avoid data
corruption is to always make the encoding of the exchanged
data explicit. Choose an encoding that you know the extension
can handle. Convert arguments passed to the extensions
to that encoding and convert results back from that
encoding. Write wrapper functions that do the conversions
for you, so you can later change the functions when the
extension catches up.
To provide an example, let's say the popular
Foo::Bar::escape_html function doesn't deal with Unicode
data yet. The wrapper function would convert the argument
to raw UTF-8 and convert the result back to Perl's internal
representation like so:
sub my_escape_html ($) {
my($what) = shift;
return unless defined $what;
Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
}
Sometimes, when the extension does not convert data but
just stores and retrieves them, you will be in a position
to use the otherwise dangerous Encode::_utf8_on() function.
Let's say the popular "Foo::Bar" extension, written
in C, provides a "param" method that lets you store and
retrieve data according to these prototypes:
$self->param($name, $value); # set a scalar
$value = $self->param($name); # retrieve a
scalar
If it does not yet provide support for any encoding, one
could write a derived class with such a "param" method:
sub param {
my($self,$name,$value) = @_;
utf8::upgrade($name); # make sure it is UTF-8
encoded
if (defined $value)
utf8::upgrade($value); # make sure it is UTF-8
encoded
return $self->SUPER::param($name,$value);
} else {
my $ret = $self->SUPER::param($name);
Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
return $ret;
}
}
Some extensions provide filters on data entry/exit points,
such as DB_File::filter_store_key and family. Look out for
such filters in the documentation of your extensions, they
can make the transition to Unicode data much easier.
Speed [Toc] [Back]
Some functions are slower when working on UTF-8 encoded
strings than on byte encoded strings. All functions that
need to hop over characters such as length(), substr() or
index(), or matching regular expressions can work much
faster when the underlying data are byte-encoded.
In Perl 5.8.0 the slowness was often quite spectacular; in
Perl 5.8.1 a caching scheme was introduced which will
hopefully make the slowness somewhat less spectacular, at
least for some operations. In general, operations with
UTF-8 encoded strings are still slower. As an example, the
Unicode properties (character classes) like "{Nd}"
are
known to be quite a bit slower (5-20 times) than their
simpler counterparts like "" (then again, there 268 Unicode
characters matching "Nd" compared with the 10 ASCII
characters matching "d").
Porting code from perl-5.6.X [Toc] [Back]
Perl 5.8 has a different Unicode model from 5.6. In 5.6
the programmer was required to use the "utf8" pragma to
declare that a given scope expected to deal with Unicode
data and had to make sure that only Unicode data were
reaching that scope. If you have code that is working with
5.6, you will need some of the following adjustments to
your code. The examples are written such that the code
will continue to work under 5.6, so you should be safe to
try them out.
o A filehandle that should read or write UTF-8
if ($] > 5.007) {
binmode $fh, ":utf8";
}
o A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension
that has no mention of Unicode in the manpage, you
need to make sure that the UTF-8 flag is stripped off.
Note that at the time of this writing (October 2002)
the mentioned modules are not UTF-8-aware. Please
check the documentation to verify if this is still
true.
if ($] > 5.007) {
require Encode;
$val = Enco
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