doc/RE

Oniguruma Regular Expressions Version 6.9.11    2025/03/21

syntax: ONIG_SYNTAX_ONIGURUMA (default syntax)


1. Syntax elements

  \       escape (enable or disable meta character)
  |       alternation
  (...)   group
  [...]   character class


2. Characters

  \t           horizontal tab         (0x09)
  \v           vertical tab           (0x0B)
  \n           newline (line feed)    (0x0A)
  \r           carriage return        (0x0D)
  \b           backspace              (0x08)
  \f           form feed              (0x0C)
  \a           bell                   (0x07)
  \e           escape                 (0x1B)
  \nnn         octal char                    (encoded byte value)
  \xHH         hexadecimal char              (encoded byte value)
  \x{7HHHHHHH} (1-8 digits) hexadecimal char (code point value)
  \o{17777777777} (1-11 digits) octal char   (code point value)
  \uHHHH       hexadecimal char              (code point value)
  \cx          control char                  (code point value)
  \C-x         control char                  (code point value)
  \M-x         meta  (x|0x80)                (code point value)
  \M-\C-x      meta control char             (code point value)

 (* \b as backspace is effective in character class only)


2.1 Code point sequences

  Hexadecimal code point  (1-8 digits)
  \x{7HHHHHHH 7HHHHHHH ... 7HHHHHHH}

  Octal code point  (1-11 digits)
  \o{17777777777 17777777777 ... 17777777777}


3. Character types

  .        any character (except newline)

  \w       word character

           Not Unicode:
             alphanumeric, "_" and multibyte char.

           Unicode:
             General_Category -- (Letter|Mark|Number|Connector_Punctuation)

  \W       non-word char

  \s       whitespace char

           Not Unicode:
             \t, \n, \v, \f, \r, \x20

           Unicode case:
             U+0009, U+000A, U+000B, U+000C, U+000D, U+0085(NEL),
             General_Category -- Line_Separator
                              -- Paragraph_Separator
                              -- Space_Separator

  \S       non-whitespace char

  \d       decimal digit char

           Unicode: General_Category -- Decimal_Number

  \D       non-decimal-digit char

  \h       hexadecimal digit char   [0-9a-fA-F]

  \H       non-hexdigit char

  \R       general newline  (* can't be used in character-class)
           "\r\n" or \n,\v,\f,\r  (* but doesn't backtrack from \r\n to \r)

           Unicode case:
             "\r\n" or \n,\v,\f,\r or U+0085, U+2028, U+2029

  \N       negative newline  (?-m:.)

  \O       true anychar      (?m:.)    (* original function)

  \X       Text Segment    \X === (?>\O(?:\Y\O)*)

           The meaning of this operator changes depending on the setting of
           the option (?y{..}).

           \X doesn't check whether matching start position is boundary or not.
           Please write as \y\X if you want to ensure it.

           [Extended Grapheme Cluster mode] (default)
             Unicode case:
               See [Unicode Standard Annex #29: http://unicode.org/reports/tr29/]

             Not Unicode case:  \X === (?>\r\n|\O)

           [Word mode]
             Currently, this mode is supported in Unicode only.
             See [Unicode Standard Annex #29: http://unicode.org/reports/tr29/]


  Character Property

    * \p{property-name}
    * \p{^property-name}    (negative)
    * \P{property-name}     (negative)
    * \pX   (X = C, L, M, N, P, S, Z)
    * \PX   (X = C, L, M, N, P, S, Z)  (negative)

    property-name:

     + works on all encodings
       Alnum, Alpha, Blank, Cntrl, Digit, Graph, Lower,
       Print, Punct, Space, Upper, XDigit, Word, ASCII

     + works on EUC_JP, Shift_JIS
       Hiragana, Katakana

     + works on UTF8, UTF16, UTF32
       See doc/UNICODE_PROPERTIES.


4. Quantifier

  greedy

    ?       1 or 0 times
    *       0 or more times
    +       1 or more times
    {n,m}   (n <= m)  at least n but no more than m times
    {n,}    at least n times
    {,n}    at least 0 but no more than n times ({0,n})
    {n}     n times


  reluctant

    ??      0 or 1 times
    *?      0 or more times
    +?      1 or more times
    {n,m}?  (n <= m)  at least n but not more than m times
    {n,}?   at least n times
    {,n}?   at least 0 but not more than n times (== {0,n}?)

      {n}? is reluctant operator in ONIG_SYNTAX_JAVA and ONIG_SYNTAX_PERL only.
      (In that case, it doesn't make sense to write so.)
      In default syntax, /a{n}?/ === /(?:a{n})?/


  possessive (greedy and does not backtrack once match)

    ?+      1 or 0 times
    *+      0 or more times
    ++      1 or more times
    {n,m}   (n > m)  at least m but not more than n times

      {n,m}+, {n,}+, {n}+ are possessive operators in ONIG_SYNTAX_JAVA and
      ONIG_SYNTAX_PERL only.

    ex. /a*+/ === /(?>a*)/


5. Anchors

  ^       beginning of the line
  $       end of the line
  \b      word boundary
  \B      non-word boundary

  \A      beginning of string
  \Z      end of string, or before newline at the end
  \z      end of string
  \G      where the current search attempt begins
  \K      keep (keep start position of the result string)


  \y      Text Segment boundary
  \Y      Text Segment non-boundary

          The meaning of these operators(\y, \Y) changes depending on the setting
          of the option (?y{..}).

          [Extended Grapheme Cluster mode] (default)
            Unicode case:
              See [Unicode Standard Annex #29: http://unicode.org/reports/tr29/]

            Not Unicode:
              All positions except between \r and \n.

          [Word mode]
            Currently, this mode is supported in Unicode only.
            See [Unicode Standard Annex #29: http://unicode.org/reports/tr29/]


6. Character class

  ^...    negative class (lowest precedence)
  x-y     range from x to y
  [...]   set (character class in character class)
  ..&&..  intersection (low precedence, only higher than ^)

    ex. [a-w&&[^c-g]z] ==> ([a-w] AND ([^c-g] OR z)) ==> [abh-w]

  * If you want to use '[', '-', or ']' as a normal character
    in character class, you should escape them with '\'.


  POSIX bracket ([:xxxxx:], negate [:^xxxxx:])

    Not Unicode Case:

      alnum    alphabet or digit char
      alpha    alphabet
      ascii    code value: [0 - 127]
      blank    \t, \x20
      cntrl
      digit    0-9
      graph    include all of multibyte encoded characters
      lower
      print    include all of multibyte encoded characters
      punct
      space    \t, \n, \v, \f, \r, \x20
      upper
      xdigit   0-9, a-f, A-F
      word     alphanumeric, "_" and multibyte characters


    Unicode Case:

      alnum    Alphabetic | Decimal_Number
      alpha    Alphabetic
      ascii    U+0000 - U+007F
      blank    Space_Separator | U+0009
      cntrl    U+0000 - U+001F, U+007F - U+009F
      digit    Decimal_Number
      graph    ^White_Space && ^[[:cntrl:]] && ^Unassigned && ^Surrogate
      lower    Lowercase
      print    [[:graph:]] | Space_Separator
      punct    Punctuation | Symbol
      space    White_Space
      upper    Uppercase
      xdigit   U+0030 - U+0039 | U+0041 - U+0046 | U+0061 - U+0066
               (0-9, a-f, A-F)
      word     Alphabetic | Mark | Decimal_Number | Connector_Punctuation


7. Extended groups

  (?#...)            comment

  (?imxWDSPy-imxWDSP:subexp)  option on/off for subexp

                           i: ignore case
                           m: multi-line (dot (.) also matches newline)
                           x: extended form
                           W: ASCII only word (\w, \p{Word}, [[:word:]])
                              ASCII only word bound (\b)
                           D: ASCII only digit (\d, \p{Digit}, [[:digit:]])
                           S: ASCII only space (\s, \p{Space}, [[:space:]])
                           P: ASCII only POSIX properties (includes W,D,S)
                              (alnum, alpha, blank, cntrl, digit, graph,
                               lower, print, punct, space, upper, xdigit, word)

                           y{?}: Text Segment mode
                              This option changes the meaning of \X, \y, \Y.
                              Currently, this option is supported in Unicode only.

                              y{g}: Extended Grapheme Cluster mode (default)
                              y{w}: Word mode
                              See [Unicode Standard Annex #29]

  (?imxWDSPy-imxWDSP)  isolated option

                      * It makes a group to the next ')' or end of the pattern.
                        /ab(?i)c|def|gh/ == /ab(?i:c|def|gh)/

  * (?i) option has no effect on word types (\w, \p{Word}). However, if the word types are used within a character class, it is valid. But, this would only be a concern when word types are used with the (?W) option.


  /(?CIL).../, /(?CIL:...)/   whole option

                            This option must be placed in a position that
                            affects the entire regular expression.

                            C: ONIG_OPTION_DONT_CAPTURE_GROUP
                            I: ONIG_OPTION_IGNORECASE_IS_ASCII
                            L: ONIG_OPTION_FIND_LONGEST

  (?:subexp)         non-capturing group
  (subexp)           capturing group

  (?=subexp)         look-ahead
  (?!subexp)         negative look-ahead

  (?<=subexp)        look-behind
  (?<!subexp)        negative look-behind

                     * Cannot use Absent stopper (?~|expr) and Range clear
                       (?~|) operators in look-behind and negative look-behind.

                     * In look-behind and negative look-behind, support for
                       ignore-case option is limited. Only supports conversion
                       between single characters. (Does not support conversion
                       of multiple characters in Unicode)

  (?>subexp)         atomic group
                     no backtracks in subexp.

  (?<name>subexp), (?'name'subexp)
                     define named group
                     (Each character of the name must be a word character.)

                     Not only a name but a number is assigned like a capturing
                     group.

                     Assigning the same name to two or more subexps is allowed.


  <Callouts>

  * Callouts of contents
  (?{...contents...})         callout in progress
  (?{...contents...}D)        D is a direction flag char
                              D = 'X': in progress and retraction
                                  '<': in retraction only
                                  '>': in progress only
  (?{...contents...}[tag])    tag assigned
  (?{...contents...}[tag]D)

                              * Escape characters have no effects in contents.
                              * contents is not allowed to start with '{'.

  (?{{{...contents...}}})     n times continuations '}' in contents is allowed in
                              (n+1) times continuations {{{...}}}.

    Allowed tag string characters: _ A-Z a-z 0-9 (* first character: _ A-Z a-z)


  * Callouts of name
  (*name)
  (*name{args...})            with args
  (*name[tag])                tag assigned
  (*name[tag]{args...})

    Allowed name string characters: _ A-Z a-z 0-9 (* first character: _ A-Z a-z)
    Allowed tag  string characters: _ A-Z a-z 0-9 (* first character: _ A-Z a-z)


  <Absent functions>

  (?~absent)         Absent repeater    (* proposed by Tanaka Akira)
                     This works like .* (more precisely \O*), but it is
                     limited by the range that does not include the string
                     match with <absent>.
                     This is a written abbreviation of (?~|(?:absent)|\O*).
                     \O* is used as a repeater.

  (?~|absent|exp)    Absent expression  (* original)
                     This works like "exp", but it is limited by the range
                     that does not include the string match with <absent>.

                     ex. (?~|345|\d*)  "12345678"  ==> "12", "1", ""

  (?~|absent)        Absent stopper (* original)
                     After passed this operator, string right range is limited
                     at the point that does not include the string match with
                     <absent>.

  (?~|)              Range clear
                     Clear the effects caused by Absent stoppers.

     * Nested Absent functions are not supported and the behavior
       is undefined.


  <if-then-else>

  (?(condition_exp)then_exp|else_exp)    if-then-else
  (?(condition_exp)then_exp)             if-then

               condition_exp can be a backreference number/name or a normal
               regular expression.
               When condition_exp is a backreference number/name, both then_exp and
               else_exp can be omitted.
               Then it works as a backreference validity checker.

  [ Backreference validity checker ]   (* original)

    (?(n)), (?(-n)), (?(+n)), (?(n+level)) ...
    (?(<n>)), (?('-n')), (?(<+n>)) ...
    (?(<name>)), (?('name')), (?(<name+level>)) ...


8. Backreferences

  When we say "backreference a group," it actually means, "re-match the same
  text matched by the subexp in that group."

  \n  \k<n>     \k'n'     (n >= 1) backreference the nth group in the regexp
      \k<-n>    \k'-n'    (n >= 1) backreference the nth group counting
                          backwards from the referring position
      \k<+n>    \k'+n'    (n >= 1) backreference the nth group counting
                          forwards from the referring position
      \k<name>  \k'name'  backreference a group with the specified name

  When backreferencing with a name that is assigned to more than one groups,
  the last group with the name is checked first, if not matched then the
  previous one with the name, and so on, until there is a match.

  * Backreference by number is forbidden if any named group is defined and
    ONIG_OPTION_CAPTURE_GROUP is not set.


  backreference with recursion level

    (n >= 1, level >= 0)

    \k<n+level> \k'n+level'
    \k<n-level> \k'n-level'

    \k<name+level> \k'name+level'
    \k<name-level> \k'name-level'

    Destine a group on the recursion level relative to the referring position.

    ex 1.

      /\A(?<a>|.|(?:(?<b>.)\g<a>\k<b>))\z/.match("reee")
      /\A(?<a>|.|(?:(?<b>.)\g<a>\k<b+0>))\z/.match("reer")

      \k<b+0> refers to the (?<b>.) on the same recursion level with it.

    ex 2.

      r = Regexp.compile(<<'__REGEXP__'.strip, Regexp::EXTENDED)
      (?<element> \g<stag> \g<content>* \g<etag> ){0}
      (?<stag> < \g<name> \s* > ){0}
      (?<name> [a-zA-Z_:]+ ){0}
      (?<content> [^<&]+ (\g<element> | [^<&]+)* ){0}
      (?<etag> </ \k<name+1> >){0}
      \g<element>
      __REGEXP__

      p r.match("<foo>f<bar>bbb</bar>f</foo>").captures


9. Subexp calls ("Tanaka Akira special")   (* original function)

  When we say "call a group," it actually means, "re-execute the subexp in
  that group."

  \g<n>     \g'n'     (n >= 1) call the nth group
  \g<0>     \g'0'     call zero (call the total regexp)
  \g<-n>    \g'-n'    (n >= 1) call the nth group counting backwards from
                      the calling position
  \g<+n>    \g'+n'    (n >= 1) call the nth group counting forwards from
                      the calling position
  \g<name>  \g'name'  call the group with the specified name

  * Left-most recursive calls are not allowed.

    ex. (?<name>a|\g<name>b)    => error
        (?<name>a|b\g<name>c)   => OK

  * Calls with a name that is assigned to more than one groups are not
    allowed.

  * Call by number is forbidden if any named group is defined and
    ONIG_OPTION_CAPTURE_GROUP is not set.

  * The option status of the called group is always effective.

    ex. /(?-i:\g<name>)(?i:(?<name>a)){0}/.match("A")


10. Captured group

  Behavior of an unnamed group (...) changes with the following conditions.
  (But named group is not changed.)

  case 1. /.../     (named group is not used, no option)

     (...) is treated as a capturing group.

  case 2. /.../g    (named group is not used, 'g' option)

     (...) is treated as a non-capturing group (?:...).

  case 3. /..(?<name>..)../   (named group is used, no option)

     (...) is treated as a non-capturing group.
     numbered-backref/call is not allowed.

  case 4. /..(?<name>..)../G  (named group is used, 'G' option)

     (...) is treated as a capturing group.
     numbered-backref/call is allowed.

  where
    g: ONIG_OPTION_DONT_CAPTURE_GROUP
    G: ONIG_OPTION_CAPTURE_GROUP

  ('g' and 'G' options are argued in ruby-dev ML)


-----------------------------
A-1. Syntax-dependent options

   + ONIG_SYNTAX_ONIGURUMA
     (?m): dot (.) also matches newline

   + ONIG_SYNTAX_PERL and ONIG_SYNTAX_JAVA
     (?s): dot (.) also matches newline
     (?m): ^ matches after newline, $ matches before newline


A-2. Original extensions

   + hexadecimal digit char type     \h, \H
   + true anychar                    \O
   + text segment boundary           \y, \Y
   + backreference validity checker  (?(...))
   + named group                     (?<name>...), (?'name'...)
   + named backref                   \k<name>
   + subexp call                     \g<name>, \g<group-num>
   + absent expression               (?~|...|...)
   + absent stopper                  (?~|...)


A-3. Missing features compared with perl 5.8.0

   + \N{name}
   + \l,\u,\L,\U,\C
   + (??{code})

   * \Q...\E
     This is effective on ONIG_SYNTAX_PERL and ONIG_SYNTAX_JAVA.


A-4. Differences with Japanized GNU regex(version 0.12) of Ruby 1.8

   + add character property (\p{property}, \P{property})
   + add hexadecimal digit char type (\h, \H)
   + add look-behind
     (?<=fixed-width-pattern), (?<!fixed-width-pattern)
   + add possessive quantifier. ?+, *+, ++
   + add operations in character class. [], &&
     ('[' must be escaped as an usual char in character class.)
   + add named group and subexp call.
   + octal or hexadecimal number sequence can be treated as
     a multibyte code char in character class if multibyte encoding
     is specified.
     (ex. [\xa1\xa2], [\xa1\xa7-\xa4\xa1])
   + allow the range of single byte char and multibyte char in character
     class.
     ex. /[a-<<any EUC-JP character>>]/ in EUC-JP encoding.
   + effect range of isolated option is to next ')'.
     ex. (?:(?i)a|b) is interpreted as (?:(?i:a|b)), not (?:(?i:a)|b).
   + isolated option is not transparent to previous pattern.
     ex. a(?i)* is a syntax error pattern.
   + allowed unpaired left brace as a normal character.
     ex. /{/, /({)/, /a{2,3/ etc...
   + negative POSIX bracket [:^xxxx:] is supported.
   + POSIX bracket [:ascii:] is added.
   + repeat of look-ahead is not allowed.
     ex. /(?=a)*/, /(?!b){5}/
   + Ignore case option is effective to escape sequence.
     ex. /\x61/i =~ "A"
   + In the range quantifier, the number of the minimum is optional.
     /a{,n}/ == /a{0,n}/
     The omission of both minimum and maximum values is not allowed.
     /a{,}/
   + /{n}?/ is not a reluctant quantifier.
     /a{n}?/ == /(?:a{n})?/
   + invalid back reference is checked and raises error.
     /\1/, /(a)\2/
   + Zero-width match in an infinite loop stops the repeat,
     then changes of the capture group status are checked as stop condition.
     /(?:()|())*\1\2/ =~ ""
     /(?:\1a|())*/ =~ "a"

// END