Onigmo (Oniguruma-mod) Regular Expressions Version 6.1.0    2016/12/25

syntax: ONIG_SYNTAX_RUBY (default)


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} wide hexadecimal char  (character code point value)
  \uHHHH       wide hexadecimal char  (character code point value)
  \cx          control char           (character code point value)
  \C-x         control char           (character code point value)
  \M-x         meta  (x|0x80)         (character code point value)
  \M-\C-x      meta control char      (character code point value)

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

  * ONIG_SYNTAX_PERL: \o{nnn} (octal char) can be also used.


3. Character types

  .        any character (except newline)

  \w       word character

           Not Unicode:
             alphanumeric and "_".

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

           It depends on ONIG_OPTION_ASCII_RANGE option that non-ASCII char
           includes or not.

  \W       non-word char

  \s       whitespace char

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

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

           It depends on ONIG_OPTION_ASCII_RANGE option that non-ASCII char
           includes or not.

  \S       non-whitespace char

  \d       decimal digit char

           Unicode: General_Category -- Decimal_Number

           It depends on ONIG_OPTION_ASCII_RANGE option that non-ASCII char
           includes or not.

  \D       non-decimal-digit char

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

  \H       non-hexadecimal-digit char


  Character Property

    * \p{property-name}
    * \p{^property-name}    (negative)
    * \P{property-name}     (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, CP932
       Hiragana, Katakana, Han, Latin, Greek, Cyrillic

     + works on UTF-8, UTF-16, UTF-32
       see UnicodeProps.txt

    \p{Punct} works slightly different on Unicode encodings and the other
    encodings. It matches the nine characters "$+<=>^`|~" on non-Unicode
    encodings (which is the same as [[:punct:]]), but not on Unicode encodings.
    \p{XPosixPunct} matches the nine characters on Unicode encodings.


  \R       Linebreak

           Unicode:
             (?>\x0D\x0A|[\x0A-\x0D\x{85}\x{2028}\x{2029}])

           Not Unicode:
             (?>\x0D\x0A|[\x0A-\x0D])

  \X       Extended Grapheme cluster

           Unicode:
             See: Unicode Standard Annex #29 UNICODE TEXT SEGMENTATION
                  http://unicode.org/reports/tr29/

           Not Unicode:
             (?>\x0D\x0A|(?m:.))



4. Quantifier

  greedy

    ?       1 or 0 times
    *       0 or more times
    +       1 or more times
    {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

    ??      1 or 0 times
    *?      0 or more times
    +?      1 or more times
    {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}?)

  possessive (greedy and does not backtrack once match)

    ?+      1 or 0 times
    *+      0 or more times
    ++      1 or more times

    ({n,m}+, {n,}+, {n}+ are possessive op. 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


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    \x21-\x7E and all of multibyte encoded characters
      lower
      print    \x20-\x7E and 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    Letter | Mark | Decimal_Number
      alpha    Letter | Mark
      ascii    0000 - 007F
      blank    Space_Separator | 0009
      cntrl    Control | Format | Unassigned | Private_Use | Surrogate
      digit    Decimal_Number
      graph    [[:^space:]] && ^Control && ^Unassigned && ^Surrogate
      lower    Lowercase_Letter
      print    [[:graph:]] | Space_Separator
      punct    Connector_Punctuation | Dash_Punctuation | Close_Punctuation |
               Final_Punctuation | Initial_Punctuation | Other_Punctuation |
               Open_Punctuation | 0024 | 002B | 003C | 003D | 003E | 005E |
               0060 | 007C | 007E
      space    Space_Separator | Line_Separator | Paragraph_Separator |
               0009 | 000A | 000B | 000C | 000D | 0085
      upper    Uppercase_Letter
      xdigit   0030 - 0039 | 0041 - 0046 | 0061 - 0066
               (0-9, a-f, A-F)
      word     Letter | Mark | Decimal_Number | Connector_Punctuation


    It depends on ONIG_OPTION_ASCII_RANGE option and
    ONIG_OPTION_POSIX_BRACKET_ALL_RANGE option that POSIX brackets
    match non-ASCII char or not.



7. Extended groups

  (?#...)            comment

  (?imxdau-imx)      option on/off
                         i: ignore case
                         m: multi-line (dot (.) also matches newline)
                         x: extended form

                       character set option (character range option)
                         d: Default (compatible with Ruby 1.9.3)
                            \w, \d and \s doesn't match non-ASCII characters.
                            \b, \B and POSIX brackets use the each encoding's
                            rules.
                         a: ASCII
                            ONIG_OPTION_ASCII_RANGE option is turned on.
                            \w, \d, \s and POSIX brackets doesn't match
                            non-ASCII characters.
                            \b and \B use the ASCII rules.
                         u: Unicode
                            ONIG_OPTION_ASCII_RANGE option is turned off.
                            \w (\W), \d (\D), \s (\S), \b (\B) and POSIX
                            brackets use the each encoding's rules.

  (?imxdau-imx:subexp)
                     option on/off for subexp

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

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

                     Subexp of look-behind must be fixed-width.
                     But top-level alternatives can be of various lengths.
                     ex. (?<=a|bc) is OK. (?<=aaa(?:b|cd)) is not allowed.

                     In negative look-behind, capturing group isn't allowed,
                     but non-capturing group (?:) is allowed.

  \K                 keep
                     Another expression of look-behind. Keep the stuff left
                     of the \K, don't include it in the result.

  (?>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.

  (?(cond)yes-subexp), (?(cond)yes-subexp|no-subexp)
                    conditional expression
                    Matches yes-subexp if (cond) yields a true value, matches
                    no-subexp otherwise.
                    Following (cond) can be used:

                    (n)  (n >= 1)
                        Checks if the numbered capturing group has matched
                        something.

                    (<name>), ('name')
                        Checks if a group with the given name has matched
                        something.

                        BUG: If the name is defined more than once, the
                        left-most group is checked, but it should be the
                        same as \k<name>.

  (?~subexp)        absence operator (experimental)
                    Matches any string which doesn't contain any string which
                    matches subexp.
                    More precisely, (?~subexp) matches the complement set of
                    a set which .*subexp.* matches. This is regular in the
                    meaning of formal language theory.
                    Similar to (?:(?!subexp).)*, but easy to write.

                    E.g.:
                      (?~abc) matches: "", "ab", "aab", "ccdd", etc.
                      It doesn't match: "abc", "aabc", "ccabcdd", etc.

                      \/\*(?~\*\/)\*\/ matches C style comments:
                        "/**/", "/* foobar */", etc.

                      \A\/\*(?~\*\/)\*\/\z doesn't match "/**/ */".
                      This is different from \A\/\*.*?\*\/\z which uses a
                      reluctant quantifier (.*?).

                      Unlike (?:(?!abc).)*c, (?~abc)c matches "abc", because
                      (?~abc) matches "ab".

                      (?~) never matches.

                    Theoretical backgrounds are discussed in Tanaka Akira's
                    paper and slide (both Japanese):

                      * Absent Operator for Regular Expression
                        https://staff.aist.go.jp/tanaka-akira/pub/prosym49-akr-paper.pdf
                      * 正規表現における非包含オペレータの提案
                        https://staff.aist.go.jp/tanaka-akira/pub/prosym49-akr-presen.pdf


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<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.

  * ONIG_SYNTAX_PERL: \g{n}, \g{-n} and \g{name} can also be used.
    If a name is defined more than once in Perl syntax, only the left-most
    group is checked.


  backreference with recursion level

    (n >= 1, level >= 0)

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

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

  \g<0>     \g'0'     call the whole pattern recursively
  \g<n>     \g'n'     (n >= 1) call the nth group
  \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 in ONIG_SYNTAX_RUBY.

  * 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")

  * ONIG_SYNTAX_PERL:
    Use (?&name), (?n), (?-n), (?+n), (?R) or (?0) instead of \g<>.
    Calls with a name that is assigned to more than one groups are allowed,
    and the left-most subexp is used.


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_RUBY
     (?m): dot (.) also matches newline

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

   + ONIG_SYNTAX_PERL
     (?d), (?l): same as (?u)


A-2. Original extensions

   + hexadecimal digit char type  \h, \H
   + named group                  (?<name>...), (?'name'...)
   + named backref                \k<name>
   + subexp call                  \g<name>, \g<group-num>


A-3. Missing features compared with perl 5.18.0

   + \N{name}, \N{U+xxxx}, \N
   + \l,\u,\L,\U, \C
   + \v, \V, \h, \H
   + (?{code})
   + (??{code})
   + (?|...)
   + (?[])
   + (*VERB:ARG)

   * \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"


A-5. Features disabled in default syntax

   + capture history

     (?@...) and (?@<name>...)

     ex. /(?@a)*/.match("aaa") ==> [<0-1>, <1-2>, <2-3>]

     see sample/listcap.c file.


A-6. Problems

   + Invalid encoding byte sequence is not checked.

     ex. UTF-8

     * Invalid first byte is treated as a character.
       /./u =~ "\xa3"

     * Incomplete byte sequence is not checked.
       /\w+/ =~ "a\xf3\x8ec"

// END
