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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 recognition of UTF-8 in the Perl scripts themselves
(in string or regular expression literals, or in identifier names) on
ASCII-based machines or to recognize UTF-EBCDIC on EBCDIC-based
machines. These are the only times when an explicit "use utf8" is
needed. See utf8.
You can also use the "encoding" pragma to change the default encoding
of the data in your script; see encoding.
"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
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
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}".
· 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 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 "\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
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. "L&" is a special case,
which is an alias for "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:
Property Meaning
BidiL Left-to-Right
BidiLRE Left-to-Right Embedding
BidiLRO Left-to-Right Override
BidiR Right-to-Left
BidiAL Right-to-Left Arabic
BidiRLE Right-to-Left Embedding
BidiRLO Right-to-Left Override
BidiPDF Pop Directional Format
BidiEN European Number
BidiES European Number Separator
BidiET European Number Terminator
BidiAN Arabic Number
BidiCS Common Number Separator
BidiNSM Non-Spacing Mark
BidiBN Boundary Neutral
BidiB Paragraph Separator
BidiS Segment Separator
BidiWS Whitespace
BidiON Other Neutrals
For example, "\p{BidiR}" 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:
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 \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" functionality 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 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()",
"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
"chomp()", 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 encoding 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 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 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 must be defined in the
"main" package. The user-defined properties can be used in the regular
expression "\p" and "\P" constructs. 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::"), 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 character property
(prefixed by "utf8::"), for 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::") 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
}
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 tabulators: 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/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
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] now scripts (see UTR#24 Script Names) in addition to blocks
[ 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, 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 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 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".
· 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 encoding,
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 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.
· 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 necessary. If your legacy code
does not explicitly use Unicode, no automatic switch-over to characters
should happen. Characters shouldn't get downgraded to bytes, either.
It is possible to accidentally mix bytes and characters, however (see
perluniintro), in which case "\w" in regular expressions might start
behaving differently. Review your code. Use warnings and the "strict"
pragma.
Unicode in Perl on EBCDIC
The way Unicode is handled on EBCDIC platforms is still experimental. On
such platforms, references to UTF-8 encoding in this document and elsewhere
should be read as meaning the UTF-EBCDIC specified in Unicode Technical
Report 16, unless ASCII vs. EBCDIC issues are specifically discussed. There
is no "utfebcdic" pragma or ":utfebcdic" layer; rather, "utf8" and ":utf8"
are reused to mean the platform's "natural" 8-bit encoding of Unicode. See
perlebcdic for more discussion of the issues.
Locales
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 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.
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?
· chmod, 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 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 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-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.
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. Modules
written completely in Perl shouldn't cause problems. Modules that directly
or indirectly access code written in other programming languages are at
risk.
For affected functions, the simple strategy to avoid data corruption is to
always make the encoding of the exchanged data explicit. Choose an encoding
that you know the extension can handle. Convert arguments passed to the
extensions to that encoding and convert results back from that encoding.
Write wrapper functions that do the conversions for you, so you can later
change the functions when the extension catches up.
To provide an example, let's say the popular Foo::Bar::escape_html function
doesn't deal with Unicode data yet. The wrapper function would convert the
argument to raw UTF-8 and convert the result back to Perl's internal
representation like so:
sub my_escape_html ($) {
my($what) = shift;
return unless defined $what;
Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
}
Sometimes, when the extension does not convert data but just stores and
retrieves them, you will be in a position to use the otherwise dangerous
Encode::_utf8_on() function. Let's say the popular "Foo::Bar" extension,
written in C, provides a "param" method that lets you store and retrieve
data according to these prototypes:
$self->param($name, $value); # set a scalar
$value = $self->param($name); # retrieve a scalar
If it does not yet provide support for any encoding, one could write a
derived class with such a "param" method:
sub param {
my($self,$name,$value) = @_;
utf8::upgrade($name); # make sure it is UTF-8 encoded
if (defined $value)
utf8::upgrade($value); # make sure it is UTF-8 encoded
return $self->SUPER::param($name,$value);
} else {
my $ret = $self->SUPER::param($name);
Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
return $ret;
}
}
Some extensions provide filters on data entry/exit points, such as
DB_File::filter_store_key and family. Look out for such filters in the
documentation of your extensions, they can make the transition to Unicode
data much easier.
Speed
Some functions are slower when working on UTF-8 encoded strings than on
byte encoded strings. All functions that need to hop over characters such
as length(), substr() or index(), or matching regular expressions can work
much faster when the underlying data are byte-encoded.
In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a
caching scheme was introduced which will hopefully make the slowness
somewhat less spectacular, at least for some operations. In general,
operations with UTF-8 encoded strings are still slower. As an example, the
Unicode properties (character classes) like "\p{Nd}" are known to be quite
a bit slower (5-20 times) than their simpler counterparts like "\d" (then
again, there 268 Unicode characters matching "Nd" compared with the 10
ASCII characters matching "d").
Porting code from perl-5.6.X
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer was
required to use the "utf8" pragma to declare that a given scope expected to
deal with Unicode data and had to make sure that only Unicode data were
reaching that scope. If you have code that is working with 5.6, you will
need some of the following adjustments to your code. The examples are
written such that the code will continue to work under 5.6, so you should
be safe to try them out.
· A filehandle that should read or write UTF-8
if ($] > 5.007) {
binmode $fh, ":utf8";
}
· A scalar that is going to be passed to some extension
Be it Compress::Zlib, Apache::Request or any extension that has no
mention of Unicode in the manpage, you need to make sure that the UTF-8
flag is stripped off. Note that at the time of this writing (October
2002) the mentioned modules are not UTF-8-aware. Please check the
documentation to verify if this is still true.
if ($] > 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
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