People who want to learn to use Unicode in Perl, should probably read the Perl Unicode tutorial before reading this reference document.
To indicate that Perl source itself is in UTF-8, use "use utf8;".
See ``Byte and Character Semantics'' for more details.
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.
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.
If you use a Unicode editor to edit your program, Unicode characters may occur directly within the literal strings in UTF-8 encoding, or UTF-16. (The former requires a BOM or "use utf8", the latter requires a BOM.)
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 all characters, but for characters under 0x100, note that Perl may use an 8 bit encoding internally, for optimization and/or backward compatibility.
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}".
See ``Unicode Character Properties'' for more details.
You can define your own character properties and use them in the regular expression with the "\p{}" or "\P{}" construct.
See ``User-Defined Character Properties'' for more details.
There is a new "U" specifier that converts between Unicode characters and code points. There is also a "W" specifier that is the equivalent of "chr"/"ord" and properly handles character values even if they are above 255.
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.
But you can also define your own mappings to be used in the lc(), lcfirst(), uc(), and ucfirst() (or their string-inlined versions).
See ``User-Defined Case Mappings'' for more details.
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 5.0.0 in July 2006.
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.
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.
Arabic
Armenian
Balinese
Bengali
Bopomofo
Braille
Buginese
Buhid
CanadianAboriginal
Cherokee
Coptic
Cuneiform
Cypriot
Cyrillic
Deseret
Devanagari
Ethiopic
Georgian
Glagolitic
Gothic
Greek
Gujarati
Gurmukhi
Han
Hangul
Hanunoo
Hebrew
Hiragana
Inherited
Kannada
Katakana
Kharoshthi
Khmer
Lao
Latin
Limbu
LinearB
Malayalam
Mongolian
Myanmar
NewTaiLue
Nko
Ogham
OldItalic
OldPersian
Oriya
Osmanya
PhagsPa
Phoenician
Runic
Shavian
Sinhala
SylotiNagri
Syriac
Tagalog
Tagbanwa
TaiLe
Tamil
Telugu
Thaana
Thai
Tibetan
Tifinagh
Ugaritic
Yi
ASCIIHexDigit
BidiControl
Dash
Deprecated
Diacritic
Extender
HexDigit
Hyphen
Ideographic
IDSBinaryOperator
IDSTrinaryOperator
JoinControl
LogicalOrderException
NoncharacterCodePoint
OtherAlphabetic
OtherDefaultIgnorableCodePoint
OtherGraphemeExtend
OtherIDStart
OtherIDContinue
OtherLowercase
OtherMath
OtherUppercase
PatternSyntax
PatternWhiteSpace
QuotationMark
Radical
SoftDotted
STerm
TerminalPunctuation
UnifiedIdeograph
VariationSelector
WhiteSpace
and there are further derived properties:
Alphabetic = Lu + Ll + Lt + Lm + Lo + Nl + OtherAlphabetic
Lowercase = Ll + OtherLowercase
Uppercase = Lu + OtherUppercase
Math = Sm + OtherMath
IDStart = Lu + Ll + Lt + Lm + Lo + Nl + OtherIDStart
IDContinue = IDStart + Mn + Mc + Nd + Pc + OtherIDContinue
DefaultIgnorableCodePoint
= OtherDefaultIgnorableCodePoint
+ Cf + Cc + Cs + Noncharacters + VariationSelector
- WhiteSpace - FFF9..FFFB (Annotation Characters)
Any = Any code points (i.e. U+0000 to U+10FFFF)
Assigned = Any non-Cn code points (i.e. synonym for \P{Cn})
Unassigned = Synonym for \p{Cn}
ASCII = ASCII (i.e. U+0000 to U+007F)
Common = Any character (or unassigned code point)
not explicitly assigned to a script
For more about scripts, see the UAX#24 ``Script Names'':
http://www.unicode.org/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:
InAegeanNumbers
InAlphabeticPresentationForms
InAncientGreekMusicalNotation
InAncientGreekNumbers
InArabic
InArabicPresentationFormsA
InArabicPresentationFormsB
InArabicSupplement
InArmenian
InArrows
InBalinese
InBasicLatin
InBengali
InBlockElements
InBopomofo
InBopomofoExtended
InBoxDrawing
InBraillePatterns
InBuginese
InBuhid
InByzantineMusicalSymbols
InCJKCompatibility
InCJKCompatibilityForms
InCJKCompatibilityIdeographs
InCJKCompatibilityIdeographsSupplement
InCJKRadicalsSupplement
InCJKStrokes
InCJKSymbolsAndPunctuation
InCJKUnifiedIdeographs
InCJKUnifiedIdeographsExtensionA
InCJKUnifiedIdeographsExtensionB
InCherokee
InCombiningDiacriticalMarks
InCombiningDiacriticalMarksSupplement
InCombiningDiacriticalMarksforSymbols
InCombiningHalfMarks
InControlPictures
InCoptic
InCountingRodNumerals
InCuneiform
InCuneiformNumbersAndPunctuation
InCurrencySymbols
InCypriotSyllabary
InCyrillic
InCyrillicSupplement
InDeseret
InDevanagari
InDingbats
InEnclosedAlphanumerics
InEnclosedCJKLettersAndMonths
InEthiopic
InEthiopicExtended
InEthiopicSupplement
InGeneralPunctuation
InGeometricShapes
InGeorgian
InGeorgianSupplement
InGlagolitic
InGothic
InGreekExtended
InGreekAndCoptic
InGujarati
InGurmukhi
InHalfwidthAndFullwidthForms
InHangulCompatibilityJamo
InHangulJamo
InHangulSyllables
InHanunoo
InHebrew
InHighPrivateUseSurrogates
InHighSurrogates
InHiragana
InIPAExtensions
InIdeographicDescriptionCharacters
InKanbun
InKangxiRadicals
InKannada
InKatakana
InKatakanaPhoneticExtensions
InKharoshthi
InKhmer
InKhmerSymbols
InLao
InLatin1Supplement
InLatinExtendedA
InLatinExtendedAdditional
InLatinExtendedB
InLatinExtendedC
InLatinExtendedD
InLetterlikeSymbols
InLimbu
InLinearBIdeograms
InLinearBSyllabary
InLowSurrogates
InMalayalam
InMathematicalAlphanumericSymbols
InMathematicalOperators
InMiscellaneousMathematicalSymbolsA
InMiscellaneousMathematicalSymbolsB
InMiscellaneousSymbols
InMiscellaneousSymbolsAndArrows
InMiscellaneousTechnical
InModifierToneLetters
InMongolian
InMusicalSymbols
InMyanmar
InNKo
InNewTaiLue
InNumberForms
InOgham
InOldItalic
InOldPersian
InOpticalCharacterRecognition
InOriya
InOsmanya
InPhagspa
InPhoenician
InPhoneticExtensions
InPhoneticExtensionsSupplement
InPrivateUseArea
InRunic
InShavian
InSinhala
InSmallFormVariants
InSpacingModifierLetters
InSpecials
InSuperscriptsAndSubscripts
InSupplementalArrowsA
InSupplementalArrowsB
InSupplementalMathematicalOperators
InSupplementalPunctuation
InSupplementaryPrivateUseAreaA
InSupplementaryPrivateUseAreaB
InSylotiNagri
InSyriac
InTagalog
InTagbanwa
InTags
InTaiLe
InTaiXuanJingSymbols
InTamil
InTelugu
InThaana
InThai
InTibetan
InTifinagh
InUgaritic
InUnifiedCanadianAboriginalSyllabics
InVariationSelectors
InVariationSelectorsSupplement
InVerticalForms
InYiRadicals
InYiSyllables
InYijingHexagramSymbols
# assuming property IsForeign defined in Lang::
package main; # property package name required
if ($txt =~ /\p{Lang::IsForeign}+/) { ... }
package Lang; # property package name not required
if ($txt =~ /\p{IsForeign}+/) { ... }
Note that the effect is compile-time and immutable once defined.
The subroutines must return a specially-formatted string, with one or more newline-separated lines. Each line must be one of the following:
For example, to define a property that covers both the Japanese syllabaries (hiragana and katakana), you can define
sub InKana {
return <<END;
3040\t309F
30A0\t30FF
END
}
Imagine that the here-doc end marker is at the beginning of the line. Now you can use "\p{InKana}" and "\P{InKana}".
You could also have used the existing block property names:
sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
END
}
Suppose you wanted to match only the allocated characters, not the raw block ranges: in other words, you want to remove the non-characters:
sub InKana {
return <<'END';
+utf8::InHiragana
+utf8::InKatakana
-utf8::IsCn
END
}
The negation is useful for defining (surprise!) negated classes.
sub InNotKana {
return <<'END';
!utf8::InHiragana
-utf8::InKatakana
+utf8::IsCn
END
}
Intersection is useful for getting the common characters matched by two (or more) classes.
sub InFooAndBar {
return <<'END';
+main::Foo
&main::Bar
END
}
It's important to remember not to use ``&'' for the first set --- that would be intersecting with nothing (resulting in an empty set).
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 case mappings: they will be used only if the scalar has been marked as having Unicode characters. Old byte-style strings will not be affected.
RL1.1 Hex Notation - done [1]
RL1.2 Properties - done [2][3]
RL1.2a Compatibility Properties - done [4]
RL1.3 Subtraction and Intersection - MISSING [5]
RL1.4 Simple Word Boundaries - done [6]
RL1.5 Simple Loose Matches - done [7]
RL1.6 Line Boundaries - MISSING [8]
RL1.7 Supplementary Code Points - done [9]
[1] \x{...}
[2] \p{...} \P{...}
[3] supports not only minimal list (general category, scripts,
Alphabetic, Lowercase, Uppercase, WhiteSpace,
NoncharacterCodePoint, DefaultIgnorableCodePoint, Any,
ASCII, Assigned), but also bidirectional types, blocks, etc.
(see L</"Unicode Character Properties">)
[4] \d \D \s \S \w \W \X [:prop:] [:^prop:]
[5] can use regular expression look-ahead [a] or
user-defined character properties [b] to emulate set operations
[6] \b \B
[7] 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.
[8] should do ^ and $ also on U+000B (\v in C), FF (\f), CR (\r),
CRLF (\r\n), NEL (U+0085), LS (U+2028), and PS (U+2029);
should also affect <>, $., and script line numbers;
should not split lines within CRLF [c] (i.e. there is no empty
line between \r and \n)
[9] UTF-8/UTF-EBDDIC used in perl allows not only U+10000 to U+10FFFF
but also beyond U+10FFFF [d]
[a] You can mimic class subtraction using lookahead. For example, what UTS#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 UTS#18 grouping, intersection, union, and removal (subtraction) syntax.
[b] '+' for union, '-' for removal (set-difference), '&' for intersection (see ``User-Defined Character Properties'')
[c] Try the ":crlf" layer (see PerlIO).
[d] Avoid "use warning 'utf8';" (or say "no warning 'utf8';") to allow U+FFFF ("\x{FFFF}").
RL2.1 Canonical Equivalents - MISSING [10][11]
RL2.2 Default Grapheme Clusters - MISSING [12][13]
RL2.3 Default Word Boundaries - MISSING [14]
RL2.4 Default Loose Matches - MISSING [15]
RL2.5 Name Properties - MISSING [16]
RL2.6 Wildcard Properties - MISSING
[10] see UAX#15 "Unicode Normalization Forms"
[11] have Unicode::Normalize but not integrated to regexes
[12] have \X but at this level . should equal that
[13] UAX#29 "Text Boundaries" considers CRLF and Hangul syllable
clusters as a single grapheme cluster.
[14] see UAX#29, Word Boundaries
[15] see UAX#21 "Case Mappings"
[16] have \N{...} but neither compute names of CJK Ideographs
and Hangul Syllables nor use a loose match [e]
[e] "\N{...}" allows namespaces (see charnames).
RL3.1 Tailored Punctuation - MISSING
RL3.2 Tailored Grapheme Clusters - MISSING [17][18]
RL3.3 Tailored Word Boundaries - MISSING
RL3.4 Tailored Loose Matches - MISSING
RL3.5 Tailored Ranges - MISSING
RL3.6 Context Matching - MISSING [19]
RL3.7 Incremental Matches - MISSING
( RL3.8 Unicode Set Sharing )
RL3.9 Possible Match Sets - MISSING
RL3.10 Folded Matching - MISSING [20]
RL3.11 Submatchers - MISSING
[17] see UAX#10 "Unicode Collation Algorithms"
[18] have Unicode::Collate but not integrated to regexes
[19] have (?<=x) and (?=x), but look-aheads or look-behinds should see
outside of the target substring
[20] need insensitive matching for linguistic features other than case;
for example, hiragana to katakana, wide and narrow, simplified Han
to traditional Han (see UTR#30 "Character Foldings")
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.
Like UTF-8 but EBCDIC-safe, in the way that UTF-8 is ASCII-safe.
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".
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.
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.
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.
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.
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.
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?
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.
For more information, see perlapi, and utf8.c and utf8.h in the Perl source code distribution.
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 $ret;
}
}
Some extensions provide filters on data entry/exit points, such as DB_File::filter_store_key and family. Look out for such filters in the documentation of your extensions, they can make the transition to Unicode data much easier.
In Perl 5.8.0 the slowness was often quite spectacular; in Perl 5.8.1 a caching scheme was introduced which will hopefully make the slowness somewhat less spectacular, at least for some operations. In general, operations with UTF-8 encoded strings are still slower. As an example, the Unicode properties (character classes) like "\p{Nd}" are known to be quite a bit slower (5-20 times) than their simpler counterparts like "\d" (then again, there 268 Unicode characters matching "Nd" compared with the 10 ASCII characters matching "d").
if ($] > 5.007) {
binmode $fh, ":encoding(utf8)";
}
Be it Compress::Zlib, Apache::Request or any extension that has no mention of Unicode in the manpage, you need to make sure that the UTF8 flag is stripped off. Note that at the time of this writing (October 2002) the mentioned modules are not UTF-8-aware. Please check the documentation to verify if this is still true.
if ($] > 5.007) {
require Encode;
$val = Encode::encode_utf8($val); # make octets
}
If you believe the scalar comes back as UTF-8, you will most likely want the UTF8 flag restored:
if ($] > 5.007) {
require Encode;
$val = Encode::decode_utf8($val);
}
if ($] > 5.007) {
require Encode;
Encode::_utf8_on($val);
}
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 $ret;
}
}
}
}
Scalars that contain only ASCII and are marked as UTF-8 are sometimes a drag to your program. If you recognize such a situation, just remove the UTF8 flag:
utf8::downgrade($val) if $] > 5.007;