perlunicode - Unicode support in Perl |
perlunicode - Unicode support in Perl
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.
To indicate that Perl source itself is using a particular encoding, see the encoding manpage.
use utf8
still needed to enable UTF-8/UTF-EBCDIC in scriptsuse 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 the utf8 manpage.
You can also use the encoding
pragma to change the default encoding
of the data in your script; see the encoding manpage.
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.
On Windows platforms, if the -C
command line switch is used or the
${^WIDE_SYSTEM_CALLS} global flag is set to 1
, all system calls
will use the corresponding wide-character APIs. This feature is
available only on Windows to conform to the API standard already
established for that platform--and there are very few non-Windows
platforms that have Unicode-aware APIs.
The bytes
pragma will always, regardless of platform, force byte
semantics in a particular lexical scope. See the bytes manpage.
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 the utf8 manpage.
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 upgraded to
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, so to change this for systems with non-Latin-1 and
non-EBCDIC native encodings use the encoding
pragma. See
the encoding manpage.
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 the perluniintro manpage for more.
Character semantics have the following effects:
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 the encoding manpage 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}
.
\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.
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}
.
Here are the basic Unicode General Category properties, followed by their
long form. You can use either; \p{Lu}
and \p{LowercaseLetter}
,
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.
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}
.
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
\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.
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:
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.
scalar reverse()
reverses by character rather than by byte.
You can define your own character properties by defining subroutines
whose names begin with ``In'' or ``Is''. The subroutines must be
visible in the package that uses the properties. The user-defined
properties can be used in the regular expression \p
and \P
constructs.
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 }
See the Encode manpage.
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''.
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 TR18 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.
[b] See User-Defined Character Properties.
Level 2 - Extended Unicode Support3.1 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 MappingsLevel 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 characters are assigned to code points, which are abstract numbers. To use these numbers, various encodings are needed.
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.
UTF-16, UTF-16BE, UTF16-LE, 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``.
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.
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 the perllocale manpage), 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 the perluniintro manpage), in which case \w
in
regular expressions might start behaving differently. Review your
code. Use warnings and the strict
pragma.
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 the perlebcdic manpage
for more discussion of the issues.
Usually locale settings and Unicode do not affect each other, but there are a couple of exceptions:
If you want to handle Perl Unicode in XS extensions, you may find the following C APIs useful. See the perlapi manpage for 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 the perlapi manpage, and utf8.c and utf8.h in the Perl source code distribution.
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.
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.
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()
can work much
faster when the underlying data are byte-encoded. Witness the
following benchmark:
% perl -e ' use Benchmark; use strict; our $l = 10000; our $u = our $b = "x" x $l; substr($u,0,1) = "\x{100}"; timethese(-2,{ LENGTH_B => q{ length($b) }, LENGTH_U => q{ length($u) }, SUBSTR_B => q{ substr($b, $l/4, $l/2) }, SUBSTR_U => q{ substr($u, $l/4, $l/2) }, }); ' Benchmark: running LENGTH_B, LENGTH_U, SUBSTR_B, SUBSTR_U for at least 2 CPU seconds... LENGTH_B: 2 wallclock secs ( 2.36 usr + 0.00 sys = 2.36 CPU) @ 5649983.05/s (n=13333960) LENGTH_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 12155.45/s (n=25648) SUBSTR_B: 3 wallclock secs ( 2.16 usr + 0.00 sys = 2.16 CPU) @ 374480.09/s (n=808877) SUBSTR_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 6791.00/s (n=14329)
The numbers show an incredible slowness on long UTF-8 strings. You should carefully avoid using these functions in tight loops. If you want to iterate over characters, the superior coding technique would split the characters into an array instead of using substr, as the following benchmark shows:
% perl -e ' use Benchmark; use strict; our $l = 10000; our $u = our $b = "x" x $l; substr($u,0,1) = "\x{100}"; timethese(-5,{ SPLIT_B => q{ for my $c (split //, $b){} }, SPLIT_U => q{ for my $c (split //, $u){} }, SUBSTR_B => q{ for my $i (0..length($b)-1){my $c = substr($b,$i,1);} }, SUBSTR_U => q{ for my $i (0..length($u)-1){my $c = substr($u,$i,1);} }, }); ' Benchmark: running SPLIT_B, SPLIT_U, SUBSTR_B, SUBSTR_U for at least 5 CPU seconds... SPLIT_B: 6 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 56.14/s (n=297) SPLIT_U: 5 wallclock secs ( 5.17 usr + 0.01 sys = 5.18 CPU) @ 55.21/s (n=286) SUBSTR_B: 5 wallclock secs ( 5.34 usr + 0.00 sys = 5.34 CPU) @ 123.22/s (n=658) SUBSTR_U: 7 wallclock secs ( 6.20 usr + 0.00 sys = 6.20 CPU) @ 0.81/s (n=5)
Even though the algorithm based on substr()
is faster than
split()
for byte-encoded data, it pales in comparison to the speed
of split()
when used with UTF-8 data.
Perl 5.8 has a different Unicode model from 5.6. In 5.6 the programmer
was required to use the utf8
pragma to declare that a given scope
expected to deal with Unicode data and had to make sure that only
Unicode data were reaching that scope. If you have code that is
working with 5.6, you will need some of the following adjustments to
your code. The examples are written such that the code will continue
to work under 5.6, so you should be safe to try them out.
if ($] > 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;
the perluniintro manpage, the encoding manpage, the Encode manpage, the open manpage, the utf8 manpage, the bytes manpage, the perlretut manpage, ${^WIDE_SYSTEM_CALLS} in the perlvar manpage
perlunicode - Unicode support in Perl |