Perl 5 version 8.0 documentation

perlunicode

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.

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.

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 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 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 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

    1. 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.

  • 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.

    1. Short Long
    2. L Letter
    3. Lu UppercaseLetter
    4. Ll LowercaseLetter
    5. Lt TitlecaseLetter
    6. Lm ModifierLetter
    7. Lo OtherLetter
    8. M Mark
    9. Mn NonspacingMark
    10. Mc SpacingMark
    11. Me EnclosingMark
    12. N Number
    13. Nd DecimalNumber
    14. Nl LetterNumber
    15. No OtherNumber
    16. P Punctuation
    17. Pc ConnectorPunctuation
    18. Pd DashPunctuation
    19. Ps OpenPunctuation
    20. Pe ClosePunctuation
    21. Pi InitialPunctuation
    22. (may behave like Ps or Pe depending on usage)
    23. Pf FinalPunctuation
    24. (may behave like Ps or Pe depending on usage)
    25. Po OtherPunctuation
    26. S Symbol
    27. Sm MathSymbol
    28. Sc CurrencySymbol
    29. Sk ModifierSymbol
    30. So OtherSymbol
    31. Z Separator
    32. Zs SpaceSeparator
    33. Zl LineSeparator
    34. Zp ParagraphSeparator
    35. C Other
    36. Cc Control
    37. Cf Format
    38. Cs Surrogate (not usable)
    39. Co PrivateUse
    40. 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:

    1. Property Meaning
    2. BidiL Left-to-Right
    3. BidiLRE Left-to-Right Embedding
    4. BidiLRO Left-to-Right Override
    5. BidiR Right-to-Left
    6. BidiAL Right-to-Left Arabic
    7. BidiRLE Right-to-Left Embedding
    8. BidiRLO Right-to-Left Override
    9. BidiPDF Pop Directional Format
    10. BidiEN European Number
    11. BidiES European Number Separator
    12. BidiET European Number Terminator
    13. BidiAN Arabic Number
    14. BidiCS Common Number Separator
    15. BidiNSM Non-Spacing Mark
    16. BidiBN Boundary Neutral
    17. BidiB Paragraph Separator
    18. BidiS Segment Separator
    19. BidiWS Whitespace
    20. 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:

  1. Arabic
  2. Armenian
  3. Bengali
  4. Bopomofo
  5. Buhid
  6. CanadianAboriginal
  7. Cherokee
  8. Cyrillic
  9. Deseret
  10. Devanagari
  11. Ethiopic
  12. Georgian
  13. Gothic
  14. Greek
  15. Gujarati
  16. Gurmukhi
  17. Han
  18. Hangul
  19. Hanunoo
  20. Hebrew
  21. Hiragana
  22. Inherited
  23. Kannada
  24. Katakana
  25. Khmer
  26. Lao
  27. Latin
  28. Malayalam
  29. Mongolian
  30. Myanmar
  31. Ogham
  32. OldItalic
  33. Oriya
  34. Runic
  35. Sinhala
  36. Syriac
  37. Tagalog
  38. Tagbanwa
  39. Tamil
  40. Telugu
  41. Thaana
  42. Thai
  43. Tibetan
  44. Yi

Extended property classes can supplement the basic properties, defined by the PropList Unicode database:

  1. ASCIIHexDigit
  2. BidiControl
  3. Dash
  4. Deprecated
  5. Diacritic
  6. Extender
  7. GraphemeLink
  8. HexDigit
  9. Hyphen
  10. Ideographic
  11. IDSBinaryOperator
  12. IDSTrinaryOperator
  13. JoinControl
  14. LogicalOrderException
  15. NoncharacterCodePoint
  16. OtherAlphabetic
  17. OtherDefaultIgnorableCodePoint
  18. OtherGraphemeExtend
  19. OtherLowercase
  20. OtherMath
  21. OtherUppercase
  22. QuotationMark
  23. Radical
  24. SoftDotted
  25. TerminalPunctuation
  26. UnifiedIdeograph
  27. WhiteSpace

and there are further derived properties:

  1. Alphabetic Lu + Ll + Lt + Lm + Lo + OtherAlphabetic
  2. Lowercase Ll + OtherLowercase
  3. Uppercase Lu + OtherUppercase
  4. Math Sm + OtherMath
  5. ID_Start Lu + Ll + Lt + Lm + Lo + Nl
  6. ID_Continue ID_Start + Mn + Mc + Nd + Pc
  7. Any Any character
  8. Assigned Any non-Cn character (i.e. synonym for \P{Cn})
  9. Unassigned Synonym for \p{Cn}
  10. Common Any character (or unassigned code point)
  11. 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:

  1. http://www.unicode.org/unicode/reports/tr24/

For more about blocks, see:

  1. 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:

  1. InAlphabeticPresentationForms
  2. InArabic
  3. InArabicPresentationFormsA
  4. InArabicPresentationFormsB
  5. InArmenian
  6. InArrows
  7. InBasicLatin
  8. InBengali
  9. InBlockElements
  10. InBopomofo
  11. InBopomofoExtended
  12. InBoxDrawing
  13. InBraillePatterns
  14. InBuhid
  15. InByzantineMusicalSymbols
  16. InCJKCompatibility
  17. InCJKCompatibilityForms
  18. InCJKCompatibilityIdeographs
  19. InCJKCompatibilityIdeographsSupplement
  20. InCJKRadicalsSupplement
  21. InCJKSymbolsAndPunctuation
  22. InCJKUnifiedIdeographs
  23. InCJKUnifiedIdeographsExtensionA
  24. InCJKUnifiedIdeographsExtensionB
  25. InCherokee
  26. InCombiningDiacriticalMarks
  27. InCombiningDiacriticalMarksforSymbols
  28. InCombiningHalfMarks
  29. InControlPictures
  30. InCurrencySymbols
  31. InCyrillic
  32. InCyrillicSupplementary
  33. InDeseret
  34. InDevanagari
  35. InDingbats
  36. InEnclosedAlphanumerics
  37. InEnclosedCJKLettersAndMonths
  38. InEthiopic
  39. InGeneralPunctuation
  40. InGeometricShapes
  41. InGeorgian
  42. InGothic
  43. InGreekExtended
  44. InGreekAndCoptic
  45. InGujarati
  46. InGurmukhi
  47. InHalfwidthAndFullwidthForms
  48. InHangulCompatibilityJamo
  49. InHangulJamo
  50. InHangulSyllables
  51. InHanunoo
  52. InHebrew
  53. InHighPrivateUseSurrogates
  54. InHighSurrogates
  55. InHiragana
  56. InIPAExtensions
  57. InIdeographicDescriptionCharacters
  58. InKanbun
  59. InKangxiRadicals
  60. InKannada
  61. InKatakana
  62. InKatakanaPhoneticExtensions
  63. InKhmer
  64. InLao
  65. InLatin1Supplement
  66. InLatinExtendedA
  67. InLatinExtendedAdditional
  68. InLatinExtendedB
  69. InLetterlikeSymbols
  70. InLowSurrogates
  71. InMalayalam
  72. InMathematicalAlphanumericSymbols
  73. InMathematicalOperators
  74. InMiscellaneousMathematicalSymbolsA
  75. InMiscellaneousMathematicalSymbolsB
  76. InMiscellaneousSymbols
  77. InMiscellaneousTechnical
  78. InMongolian
  79. InMusicalSymbols
  80. InMyanmar
  81. InNumberForms
  82. InOgham
  83. InOldItalic
  84. InOpticalCharacterRecognition
  85. InOriya
  86. InPrivateUseArea
  87. InRunic
  88. InSinhala
  89. InSmallFormVariants
  90. InSpacingModifierLetters
  91. InSpecials
  92. InSuperscriptsAndSubscripts
  93. InSupplementalArrowsA
  94. InSupplementalArrowsB
  95. InSupplementalMathematicalOperators
  96. InSupplementaryPrivateUseAreaA
  97. InSupplementaryPrivateUseAreaB
  98. InSyriac
  99. InTagalog
  100. InTagbanwa
  101. InTags
  102. InTamil
  103. InTelugu
  104. InThaana
  105. InThai
  106. InTibetan
  107. InUnifiedCanadianAboriginalSyllabics
  108. InVariationSelectors
  109. InYiRadicals
  110. 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.

    The following cases do not yet work:

    • the "final sigma" (Greek), and

    • anything to with locales (Lithuanian, Turkish, Azeri).

    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 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:

  • 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

  1. sub InKana {
  2. return <<END;
  3. 3040\t309F
  4. 30A0\t30FF
  5. END
  6. }

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:

  1. sub InKana {
  2. return <<'END';
  3. +utf8::InHiragana
  4. +utf8::InKatakana
  5. END
  6. }

Suppose you wanted to match only the allocated characters, not the raw block ranges: in other words, you want to remove the non-characters:

  1. sub InKana {
  2. return <<'END';
  3. +utf8::InHiragana
  4. +utf8::InKatakana
  5. -utf8::IsCn
  6. END
  7. }

The negation is useful for defining (surprise!) negated classes.

  1. sub InNotKana {
  2. return <<'END';
  3. !utf8::InHiragana
  4. -utf8::InKatakana
  5. +utf8::IsCn
  6. END
  7. }

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".

  • Level 1 - Basic Unicode Support

    1. 2.1 Hex Notation - done [1]
    2. Named Notation - done [2]
    3. 2.2 Categories - done [3][4]
    4. 2.3 Subtraction - MISSING [5][6]
    5. 2.4 Simple Word Boundaries - done [7]
    6. 2.5 Simple Loose Matches - done [8]
    7. 2.6 End of Line - MISSING [9][10]
    8. [ 1] \x{...}
    9. [ 2] \N{...}
    10. [ 3] . \p{...} \P{...}
    11. [ 4] now scripts (see UTR#24 Script Names) in addition to blocks
    12. [ 5] have negation
    13. [ 6] can use regular expression look-ahead [a]
    14. or user-defined character properties [b] to emulate subtraction
    15. [ 7] include Letters in word characters
    16. [ 8] note that Perl does Full case-folding in matching, not Simple:
    17. for example U+1F88 is equivalent with U+1F000 U+03B9,
    18. not with 1F80. This difference matters for certain Greek
    19. capital letters with certain modifiers: the Full case-folding
    20. decomposes the letter, while the Simple case-folding would map
    21. it to a single character.
    22. [ 9] see UTR#13 Unicode Newline Guidelines
    23. [10] should do ^ and $ also on \x{85}, \x{2028} and \x{2029})
    24. (should also affect <>, $., and script line numbers)
    25. (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

    1. [{Greek}-[{UNASSIGNED}]]

    in Perl can be written as:

    1. (?!\p{Unassigned})\p{InGreekAndCoptic}
    2. (?=\p{Assigned})\p{InGreekAndCoptic}

    But in this particular example, you probably really want

    1. \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 Support

    1. 3.1 Surrogates - MISSING
    2. 3.2 Canonical Equivalents - MISSING [11][12]
    3. 3.3 Locale-Independent Graphemes - MISSING [13]
    4. 3.4 Locale-Independent Words - MISSING [14]
    5. 3.5 Locale-Independent Loose Matches - MISSING [15]
    6. [11] see UTR#15 Unicode Normalization
    7. [12] have Unicode::Normalize but not integrated to regexes
    8. [13] have \X but at this level . should equal that
    9. [14] need three classes, not just \w and \W
    10. [15] see UTR#21 Case Mappings
  • Level 3 - Locale-Sensitive Support

    1. 4.1 Locale-Dependent Categories - MISSING
    2. 4.2 Locale-Dependent Graphemes - MISSING [16][17]
    3. 4.3 Locale-Dependent Words - MISSING
    4. 4.4 Locale-Dependent Loose Matches - MISSING
    5. 4.5 Locale-Dependent Ranges - MISSING
    6. [16] see UTR#10 Unicode Collation Algorithms
    7. [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.

    1. Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
    2. U+0000..U+007F 00..7F
    3. U+0080..U+07FF C2..DF 80..BF
    4. U+0800..U+0FFF E0 A0..BF 80..BF
    5. U+1000..U+CFFF E1..EC 80..BF 80..BF
    6. U+D000..U+D7FF ED 80..9F 80..BF
    7. U+D800..U+DFFF ******* ill-formed *******
    8. U+E000..U+FFFF EE..EF 80..BF 80..BF
    9. U+10000..U+3FFFF F0 90..BF 80..BF 80..BF
    10. U+40000..U+FFFFF F1..F3 80..BF 80..BF 80..BF
    11. 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:

    1. Code Points 1st Byte 2nd Byte 3rd Byte 4th Byte
    2. 0aaaaaaa 0aaaaaaa
    3. 00000bbbbbaaaaaa 110bbbbb 10aaaaaa
    4. ccccbbbbbbaaaaaa 1110cccc 10bbbbbb 10aaaaaa
    5. 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, 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

    1. $hi = ($uni - 0x10000) / 0x400 + 0xD800;
    2. $lo = ($uni - 0x10000) % 0x400 + 0xDC00;

    and the decoding is

    1. $uni = 0x10000 + ($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, UTF32-LE

    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:

  • If your locale environment variables (LANGUAGE, LC_ALL, LC_CTYPE, LANG) contain the strings 'UTF-8' or 'UTF8' (case-insensitive matching), the default encodings of your STDIN, STDOUT, and STDERR, and of any subsequent file open, are considered to be UTF-8.

  • 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.

Using Unicode in XS

If you want to handle Perl Unicode in XS extensions, you may find the following C APIs useful. See perlapi 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 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:

  1. sub my_escape_html ($) {
  2. my($what) = shift;
  3. return unless defined $what;
  4. Encode::decode_utf8(Foo::Bar::escape_html(Encode::encode_utf8($what)));
  5. }

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:

  1. $self->param($name, $value); # set a scalar
  2. $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:

  1. sub param {
  2. my($self,$name,$value) = @_;
  3. utf8::upgrade($name); # make sure it is UTF-8 encoded
  4. if (defined $value)
  5. utf8::upgrade($value); # make sure it is UTF-8 encoded
  6. return $self->SUPER::param($name,$value);
  7. } else {
  8. my $ret = $self->SUPER::param($name);
  9. Encode::_utf8_on($ret); # we know, it is UTF-8 encoded
  10. return $ret;
  11. }
  12. }

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() can work much faster when the underlying data are byte-encoded. Witness the following benchmark:

  1. % perl -e '
  2. use Benchmark;
  3. use strict;
  4. our $l = 10000;
  5. our $u = our $b = "x" x $l;
  6. substr($u,0,1) = "\x{100}";
  7. timethese(-2,{
  8. LENGTH_B => q{ length($b) },
  9. LENGTH_U => q{ length($u) },
  10. SUBSTR_B => q{ substr($b, $l/4, $l/2) },
  11. SUBSTR_U => q{ substr($u, $l/4, $l/2) },
  12. });
  13. '
  14. Benchmark: running LENGTH_B, LENGTH_U, SUBSTR_B, SUBSTR_U for at least 2 CPU seconds...
  15. LENGTH_B: 2 wallclock secs ( 2.36 usr + 0.00 sys = 2.36 CPU) @ 5649983.05/s (n=13333960)
  16. LENGTH_U: 2 wallclock secs ( 2.11 usr + 0.00 sys = 2.11 CPU) @ 12155.45/s (n=25648)
  17. SUBSTR_B: 3 wallclock secs ( 2.16 usr + 0.00 sys = 2.16 CPU) @ 374480.09/s (n=808877)
  18. 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:

  1. % perl -e '
  2. use Benchmark;
  3. use strict;
  4. our $l = 10000;
  5. our $u = our $b = "x" x $l;
  6. substr($u,0,1) = "\x{100}";
  7. timethese(-5,{
  8. SPLIT_B => q{ for my $c (split //, $b){} },
  9. SPLIT_U => q{ for my $c (split //, $u){} },
  10. SUBSTR_B => q{ for my $i (0..length($b)-1){my $c = substr($b,$i,1);} },
  11. SUBSTR_U => q{ for my $i (0..length($u)-1){my $c = substr($u,$i,1);} },
  12. });
  13. '
  14. Benchmark: running SPLIT_B, SPLIT_U, SUBSTR_B, SUBSTR_U for at least 5 CPU seconds...
  15. SPLIT_B: 6 wallclock secs ( 5.29 usr + 0.00 sys = 5.29 CPU) @ 56.14/s (n=297)
  16. SPLIT_U: 5 wallclock secs ( 5.17 usr + 0.01 sys = 5.18 CPU) @ 55.21/s (n=286)
  17. SUBSTR_B: 5 wallclock secs ( 5.34 usr + 0.00 sys = 5.34 CPU) @ 123.22/s (n=658)
  18. 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.

SEE ALSO

perluniintro, encoding, Encode, open, utf8, bytes, perlretut, ${^WIDE_SYSTEM_CALLS} in perlvar