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Unicode Regular Expressions

Unicode is a character set that aims to define all characters and glyphs from all human languages, living and dead. With more and more software being required to support multiple languages, or even just any language, Unicode has been strongly gaining popularity in recent years. Using different character sets for different languages is simply too cumbersome for programmers and users.

Unfortunately, Unicode brings its own requirements and pitfalls when it comes to regular expressions. Of the regex flavors discussed in this tutorial, Java, XML and the .NET framework use Unicode-based regex engines. Perl supports Unicode starting with version 5.6. PCRE can optionally be compiled with Unicode support. Note that PCRE is far less flexible in what it allows for the \p tokens, despite its name "Perl-compatible". The PHP preg functions, which are based on PCRE, support Unicode when the /u option is appended to the regular expression. Ruby supports Unicode escapes and properties in regular expressions starting with version 1.9. XRegExp brings support for Unicode properties to JavaScript.

RegexBuddy's regex engine is fully Unicode-based starting with version 2.0.0. RegexBuddy 1.x.x did not support Unicode at all. PowerGREP uses the same Unicode regex engine starting with version 3.0.0. Earlier versions would convert Unicode files to ANSI prior to grepping with an 8-bit (i.e. non-Unicode) regex engine. EditPad Pro supports Unicode starting with version 6.0.0.

Characters, Code Points, and Graphemes or How Unicode Makes a Mess of Things

Most people would consider à a single character. Unfortunately, it need not be depending on the meaning of the word "character".

All Unicode regex engines discussed in this tutorial treat any single Unicode code point as a single character. When this tutorial tells you that the dot matches any single character, this translates into Unicode parlance as "the dot matches any single Unicode code point". In Unicode, à can be encoded as two code points: U+0061 (a) followed by U+0300 (grave accent). In this situation, . applied to à will match a without the accent. ^.$ will fail to match, since the string consists of two code points. ^..$ matches à.

The Unicode code point U+0300 (grave accent) is a combining mark. Any code point that is not a combining mark can be followed by any number of combining marks. This sequence, like U+0061 U+0300 above, is displayed as a single grapheme on the screen.

Unfortunately, à can also be encoded with the single Unicode code point U+00E0 (a with grave accent). The reason for this duality is that many historical character sets encode "a with grave accent" as a single character. Unicode's designers thought it would be useful to have a one-on-one mapping with popular legacy character sets, in addition to the Unicode way of separating marks and base letters (which makes arbitrary combinations not supported by legacy character sets possible).

How to Match a Single Unicode Grapheme

Matching a single grapheme, whether it's encoded as a single code point, or as multiple code points using combining marks, is easy in Perl, PCRE, PHP, and Ruby 2.0: simply use \X. You can consider \X the Unicode version of the dot. There is one difference, though: \X always matches line break characters, whereas the dot does not match line break characters unless you enable the dot matches newline matching mode.

In Java, .NET, Ruby 1.9, and XRegExp, you can use \P{M}\p{M}*+ or (?>\P{M}\p{M}*) as a reasonably close substitute. To match any number of graphemes, use (?>\P{M}\p{M}*)+ as a substitute for \X+.

Matching a Specific Code Point

To match a specific Unicode code point, use \uFFFF where FFFF is the hexadecimal number of the code point you want to match. You must always specify 4 hexadecimal digits E.g. \u00E0 matches à, but only when encoded as a single code point U+00E0.

Perl and PCRE do not support the \uFFFF syntax. They use \x{FFFF} instead. You can omit leading zeros in the hexadecimal number between the curly braces. Since \x by itself is not a valid regex token, \x{1234} can never be confused to match \x 1234 times. It always matches the Unicode code point U+1234. \x{1234}{5678} will try to match code point U+1234 exactly 5678 times.

In Java, the regex token \uFFFF only matches the specified code point, even when you turned on canonical equivalence. However, the same syntax \uFFFF is also used to insert Unicode characters into literal strings in the Java source code. Pattern.compile("\u00E0") will match both the single-code-point and double-code-point encodings of à, while Pattern.compile("\\u00E0") matches only the single-code-point version. Remember that when writing a regex as a Java string literal, backslashes must be escaped. The former Java code compiles the regex à, while the latter compiles \u00E0. Depending on what you're doing, the difference may be significant.

JavaScript, which does not offer any Unicode support through its RegExp class, does support \uFFFF for matching a single Unicode code point as part of its string syntax.

XML Schema and XPath do not have a regex token for matching Unicode code points. However, you can easily use XML entities like  to insert literal code points into your regular expression.

Unicode Categories

In addition to complications, Unicode also brings new possibilities. One is that each Unicode character belongs to a certain category. You can match a single character belonging to the "letter" category with \p{L}. You can match a single character not belonging to that category with \P{L}.

Again, "character" really means "Unicode code point". \p{L} matches a single code point in the category "letter". If your input string is à encoded as U+0061 U+0300, it matches a without the accent. If the input is à encoded as U+00E0, it matches à with the accent. The reason is that both the code points U+0061 (a) and U+00E0 (à) are in the category "letter", while U+0300 is in the category "mark".

You should now understand why \P{M}\p{M}*+ is the equivalent of \X. \P{M} matches a code point that is not a combining mark, while \p{M}*+ matches zero or more code points that are combining marks. To match a letter including any diacritics, use \p{L}\p{M}*+. This last regex will always match à, regardless of how it is encoded. The possessive quantifier makes sure that backtracking doesn't cause \P{M}\p{M}*+ to match a non-mark without the combining marks that follow it, which \X would never do.

PCRE, PHP, and .NET are case sensitive when it checks the part between curly braces of a \p token. \p{Zs} will match any kind of space character, while \p{zs} will throw an error. All other regex engines described in this tutorial will match the space in both cases, ignoring the case of the category between the curly braces. Still, I recommend you make a habit of using the same uppercase and lowercase combination as I did in the list of properties below. This will make your regular expressions work with all Unicode regex engines.

In addition to the standard notation, \p{L}, Java, Perl, PCRE and the JGsoft engine allow you to use the shorthand \pL. The shorthand only works with single-letter Unicode properties. \pLl is not the equivalent of \p{Ll}. It is the equivalent of \p{L}l which matches Al or àl or any Unicode letter followed by a literal l.

Perl, XRegExp, and the JGsoft engine also support the longhand \p{Letter}. You can find a complete list of all Unicode properties below. You may omit the underscores or use hyphens or spaces instead.

Unicode Scripts

The Unicode standard places each assigned code point (character) into one script. A script is a group of code points used by a particular human writing system. Some scripts like Thai correspond with a single human language. Other scripts like Latin span multiple languages.

Some languages are composed of multiple scripts. There is no Japanese Unicode script. Instead, Unicode offers the Hiragana, Katakana, Han, and Latin scripts that Japanese documents are usually composed of.

A special script is the Common script. This script contains all sorts of characters that are common to a wide range of scripts. It includes all sorts of punctuation, whitespace and miscellaneous symbols.

All assigned Unicode code points (those matched by \P{Cn}) are part of exactly one Unicode script. All unassigned Unicode code points (those matched by \p{Cn}) are not part of any Unicode script at all.

The JGsoft engine, Perl, PCRE, PHP, Ruby 1.9, Delphi, and XRegExp can match Unicode scripts. Here's a list:

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

Perl and the JGsoft flavor allow you to use \p{IsLatin} instead of \p{Latin}. The "Is" syntax is useful for distinguishing between scripts and blocks, as explained in the next section. PCRE, PHP, and XRegEx do not support the "Is" prefix.

Java 7 adds support for Unicode scripts. Unlike the other flavors, Java 7 requires the "Is" prefix.

Unicode Blocks

The Unicode standard divides the Unicode character map into different blocks or ranges of code points. Each block is used to define characters of a particular script like "Tibetan" or belonging to a particular group like "Braille Patterns". Most blocks include unassigned code points, reserved for future expansion of the Unicode standard.

Note that Unicode blocks do not correspond 100% with scripts. An essential difference between blocks and scripts is that a block is a single contiguous range of code points, as listed below. Scripts consist of characters taken from all over the Unicode character map. Blocks may include unassigned code points (i.e. code points matched by \p{Cn}). Scripts never include unassigned code points. Generally, if you're not sure whether to use a Unicode script or Unicode block, use the script.

For example, the Currency block does not include the dollar and yen symbols. Those are found in the Basic_Latin and Latin-1_Supplement blocks instead, even though both are currency symbols, and the yen symbol is not a Latin character. This is for historical reasons, because the ASCII standard includes the dollar sign, and the ISO-8859 standard includes the yen sign. You should not blindly use any of the blocks listed below based on their names. Instead, look at the ranges of characters they actually match. A tool like RegexBuddy can be very helpful with this. The Unicode property \p{Sc} or \p{Currency_Symbol} would be a better choice than the Unicode block \p{InCurrency_Symbols} when trying to find all currency symbols.

  1. \p{InBasic_Latin}: U+0000–U+007F
  2. \p{InLatin-1_Supplement}: U+0080–U+00FF
  3. \p{InLatin_Extended-A}: U+0100–U+017F
  4. \p{InLatin_Extended-B}: U+0180–U+024F
  5. \p{InIPA_Extensions}: U+0250–U+02AF
  6. \p{InSpacing_Modifier_Letters}: U+02B0–U+02FF
  7. \p{InCombining_Diacritical_Marks}: U+0300–U+036F
  8. \p{InGreek_and_Coptic}: U+0370–U+03FF
  9. \p{InCyrillic}: U+0400–U+04FF
  10. \p{InCyrillic_Supplementary}: U+0500–U+052F
  11. \p{InArmenian}: U+0530–U+058F
  12. \p{InHebrew}: U+0590–U+05FF
  13. \p{InArabic}: U+0600–U+06FF
  14. \p{InSyriac}: U+0700–U+074F
  15. \p{InThaana}: U+0780–U+07BF
  16. \p{InDevanagari}: U+0900–U+097F
  17. \p{InBengali}: U+0980–U+09FF
  18. \p{InGurmukhi}: U+0A00–U+0A7F
  19. \p{InGujarati}: U+0A80–U+0AFF
  20. \p{InOriya}: U+0B00–U+0B7F
  21. \p{InTamil}: U+0B80–U+0BFF
  22. \p{InTelugu}: U+0C00–U+0C7F
  23. \p{InKannada}: U+0C80–U+0CFF
  24. \p{InMalayalam}: U+0D00–U+0D7F
  25. \p{InSinhala}: U+0D80–U+0DFF
  26. \p{InThai}: U+0E00–U+0E7F
  27. \p{InLao}: U+0E80–U+0EFF
  28. \p{InTibetan}: U+0F00–U+0FFF
  29. \p{InMyanmar}: U+1000–U+109F
  30. \p{InGeorgian}: U+10A0–U+10FF
  31. \p{InHangul_Jamo}: U+1100–U+11FF
  32. \p{InEthiopic}: U+1200–U+137F
  33. \p{InCherokee}: U+13A0–U+13FF
  34. \p{InUnified_Canadian_Aboriginal_Syllabics}: U+1400–U+167F
  35. \p{InOgham}: U+1680–U+169F
  36. \p{InRunic}: U+16A0–U+16FF
  37. \p{InTagalog}: U+1700–U+171F
  38. \p{InHanunoo}: U+1720–U+173F
  39. \p{InBuhid}: U+1740–U+175F
  40. \p{InTagbanwa}: U+1760–U+177F
  41. \p{InKhmer}: U+1780–U+17FF
  42. \p{InMongolian}: U+1800–U+18AF
  43. \p{InLimbu}: U+1900–U+194F
  44. \p{InTai_Le}: U+1950–U+197F
  45. \p{InKhmer_Symbols}: U+19E0–U+19FF
  46. \p{InPhonetic_Extensions}: U+1D00–U+1D7F
  47. \p{InLatin_Extended_Additional}: U+1E00–U+1EFF
  48. \p{InGreek_Extended}: U+1F00–U+1FFF
  49. \p{InGeneral_Punctuation}: U+2000–U+206F
  50. \p{InSuperscripts_and_Subscripts}: U+2070–U+209F
  51. \p{InCurrency_Symbols}: U+20A0–U+20CF
  52. \p{InCombining_Diacritical_Marks_for_Symbols}: U+20D0–U+20FF
  53. \p{InLetterlike_Symbols}: U+2100–U+214F
  54. \p{InNumber_Forms}: U+2150–U+218F
  55. \p{InArrows}: U+2190–U+21FF
  56. \p{InMathematical_Operators}: U+2200–U+22FF
  57. \p{InMiscellaneous_Technical}: U+2300–U+23FF
  58. \p{InControl_Pictures}: U+2400–U+243F
  59. \p{InOptical_Character_Recognition}: U+2440–U+245F
  60. \p{InEnclosed_Alphanumerics}: U+2460–U+24FF
  61. \p{InBox_Drawing}: U+2500–U+257F
  62. \p{InBlock_Elements}: U+2580–U+259F
  63. \p{InGeometric_Shapes}: U+25A0–U+25FF
  64. \p{InMiscellaneous_Symbols}: U+2600–U+26FF
  65. \p{InDingbats}: U+2700–U+27BF
  66. \p{InMiscellaneous_Mathematical_Symbols-A}: U+27C0–U+27EF
  67. \p{InSupplemental_Arrows-A}: U+27F0–U+27FF
  68. \p{InBraille_Patterns}: U+2800–U+28FF
  69. \p{InSupplemental_Arrows-B}: U+2900–U+297F
  70. \p{InMiscellaneous_Mathematical_Symbols-B}: U+2980–U+29FF
  71. \p{InSupplemental_Mathematical_Operators}: U+2A00–U+2AFF
  72. \p{InMiscellaneous_Symbols_and_Arrows}: U+2B00–U+2BFF
  73. \p{InCJK_Radicals_Supplement}: U+2E80–U+2EFF
  74. \p{InKangxi_Radicals}: U+2F00–U+2FDF
  75. \p{InIdeographic_Description_Characters}: U+2FF0–U+2FFF
  76. \p{InCJK_Symbols_and_Punctuation}: U+3000–U+303F
  77. \p{InHiragana}: U+3040–U+309F
  78. \p{InKatakana}: U+30A0–U+30FF
  79. \p{InBopomofo}: U+3100–U+312F
  80. \p{InHangul_Compatibility_Jamo}: U+3130–U+318F
  81. \p{InKanbun}: U+3190–U+319F
  82. \p{InBopomofo_Extended}: U+31A0–U+31BF
  83. \p{InKatakana_Phonetic_Extensions}: U+31F0–U+31FF
  84. \p{InEnclosed_CJK_Letters_and_Months}: U+3200–U+32FF
  85. \p{InCJK_Compatibility}: U+3300–U+33FF
  86. \p{InCJK_Unified_Ideographs_Extension_A}: U+3400–U+4DBF
  87. \p{InYijing_Hexagram_Symbols}: U+4DC0–U+4DFF
  88. \p{InCJK_Unified_Ideographs}: U+4E00–U+9FFF
  89. \p{InYi_Syllables}: U+A000–U+A48F
  90. \p{InYi_Radicals}: U+A490–U+A4CF
  91. \p{InHangul_Syllables}: U+AC00–U+D7AF
  92. \p{InHigh_Surrogates}: U+D800–U+DB7F
  93. \p{InHigh_Private_Use_Surrogates}: U+DB80–U+DBFF
  94. \p{InLow_Surrogates}: U+DC00–U+DFFF
  95. \p{InPrivate_Use_Area}: U+E000–U+F8FF
  96. \p{InCJK_Compatibility_Ideographs}: U+F900–U+FAFF
  97. \p{InAlphabetic_Presentation_Forms}: U+FB00–U+FB4F
  98. \p{InArabic_Presentation_Forms-A}: U+FB50–U+FDFF
  99. \p{InVariation_Selectors}: U+FE00–U+FE0F
  100. \p{InCombining_Half_Marks}: U+FE20–U+FE2F
  101. \p{InCJK_Compatibility_Forms}: U+FE30–U+FE4F
  102. \p{InSmall_Form_Variants}: U+FE50–U+FE6F
  103. \p{InArabic_Presentation_Forms-B}: U+FE70–U+FEFF
  104. \p{InHalfwidth_and_Fullwidth_Forms}: U+FF00–U+FFEF
  105. \p{InSpecials}: U+FFF0–U+FFFF

Not all Unicode regex engines use the same syntax to match Unicode blocks. Java, Ruby 2.0, and XRegExp use the \p{InBlock} syntax as listed above. .NET and XML use \p{IsBlock} instead. Perl and the JGsoft flavor support both notations. I recommend you use the "In" notation if your regex engine supports it. "In" can only be used for Unicode blocks, while "Is" can also be used for Unicode properties and scripts, depending on the regular expression flavor you're using. By using "In", it's obvious you're matching a block and not a similarly named property or script.

In .NET and XML, you must omit the underscores but keep the hyphens in the block names. E.g. Use \p{IsLatinExtended-A} instead of \p{InLatin_Extended-A}. In Java, you must omit the hyphens. .NET and XML also compare the names case sensitively, while Perl, Ruby, and the JGsoft flavor compare them case insensitively. Java 4 is case sensitive. Java 5 and later are case sensitive for the "Is" prefix but not for the block names themselves.

The actual names of the blocks are the same in all regular expression engines. The block names are defined in the Unicode standard. PCRE and PHP do not support Unicode blocks, even though they support Unicode scripts.

Do You Need To Worry About Different Encodings?

While you should always keep in mind the pitfalls created by the different ways in which accented characters can be encoded, you don't always have to worry about them. If you know that your input string and your regex use the same style, then you don't have to worry about it at all. This process is called Unicode normalization. All programming languages with native Unicode support, such as Java, C# and VB.NET, have library routines for normalizing strings. If you normalize both the subject and regex before attempting the match, there won't be any inconsistencies.

If you are using Java, you can pass the CANON_EQ flag as the second parameter to Pattern.compile(). This tells the Java regex engine to consider canonically equivalent characters as identical. The regex à encoded as U+00E0 matches à encoded as U+0061 U+0300, and vice versa. None of the other regex engines currently support canonical equivalence while matching.

If you type the à key on the keyboard, all word processors that I know of will insert the code point U+00E0 into the file. So if you're working with text that you typed in yourself, any regex that you type in yourself will match in the same way.

Finally, if you're using PowerGREP to search through text files encoded using a traditional Windows (often called "ANSI") or ISO-8859 code page, PowerGREP always uses the one-on-one substitution. Since all the Windows or ISO-8859 code pages encode accented characters as a single code point, nearly all software uses a single Unicode code point for each character when converting the file to Unicode.

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Regex Tutorial
Introduction
Table of Contents
Special Characters
Non-Printable Characters
Regex Engine Internals
Character Classes
Character Class Subtraction
Character Class Intersection
Shorthand Character Classes
Dot
Anchors
Word Boundaries
Alternation
Optional Items
Repetition
Grouping & Capturing
Backreferences
Backreferences, part 2
Named Groups
Branch Reset Groups
Free-Spacing & Comments
Unicode
Mode Modifiers
Atomic Grouping
Possessive Quantifiers
Lookahead & Lookbehind
Lookaround, part 2
Keep Text out of The Match
Conditionals
Balancing Groups
Recursion
Subroutines
Recursion & Capturing
Recursion & Backreferences
Recursion & Backtracking
POSIX Bracket Expressions
Zero-Length Matches
Continuing Matches
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PowerGREP 4
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