Oniguruma Regular Expressions     2004/04/27

syntax: ONIG_SYNTAX_RUBY (default)


1. Syntax elements

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


2. Characters

  \t           horizontal tab (0x09)
  \v           vertical tab   (0x0B)
  \n           newline        (0x0A)
  \r           return         (0x0D)
  \b           back space     (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)
  \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 is effective in character class [...] only)


3. Character types

  .        any character (except newline)
  \w       word character
           (alphanumeric, "_" and multibyte char. See also "A-6. Problems")
  \W       non-word char
  \s       whitespace char (\t, \n, \v, \f, \r, \x20)
  \S       non-whitespace char
  \d       digit char
  \D       non-digit char


4. Quantifier

  greedy

    ?       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})
    {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}?)
    {n}?    n times (no meaning, but same as Perl)

  possessive (greedy and does not backtrack after repeated)

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

    ({n,m}+, {n,}+, {n}+ are possessive op. in ONIG_SYNTAX_JAVA only)

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


5. Anchors

  ^       beginning of the line
  $       end of the line
  \b      word boundary
  \B      not word boundary
  \A      beginning of string
  \Z      end of string, or before newline at the end
  \z      end of string
  \G      previous end-of-match position


6. Character class

  ^...    negative class (lowest precedence operator)
  x-y     range from x to y
  [...]   set (character class in character class)
  ..&&..  intersection (low precedence at the next of ^)
          
    ex. [a-w&&[^c-g]z] ==> ([a-w] AND ([^c-g] OR z)) ==> [abh-w]

  * If you want to use '[', '-', ']' as a normal character
    in a character class, you should escape these characters by '\'.


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

    alnum    alphabet or digit char
    alpha    alphabet
    ascii    code value: [0 - 127]
    blank    \t, \x20
    cntrl
    digit    0-9
    graph
    lower
    print
    punct
    space    \t, \n, \v, \f, \r, \x20
    upper
    xdigit   0-9, a-f, A-F


7. Extended groups

  (?#...)            comment

  (?imx-imx)         option on/off
                         i: ignore case
                         m: multi-line (dot(.) match newline)
                         x: extended form
  (?imx-imx:subexp)  option on/off for subexp

  (?:subexp)         not captured group
  (subexp)           captured group

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

                     Subexp of look-behind must be fixed character length.
                     But different character length is allowed in top level
                     alternatives only.
                     ex. (?<=a|bc) is OK. (?<=aaa(?:b|cd)) is not allowed.

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

  (?>subexp)         atomic group
                     don't backtrack in subexp.

  (?<name>subexp)    define named group
                     (All characters of the name must be a word character.
                     And first character must not be a digit or uppper case)

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

                     Assigning the same name as two or more subexps is allowed.
                     In this case, a subexp call can not be performed although
                     the back reference is possible.


8. Back reference

  \n          back reference by group number (n >= 1)
  \k<name>    back reference by group name

  In the back reference by the multiplex definition name,
  a subexp with a large number is referred to preferentially.
  (When not matched, a group of the small number is referred to.)

  * Back reference by group number is forbidden if named group is defined 
    in the pattern and ONIG_OPTION_CAPTURE_GROUP is not setted.


9. Subexp call ("Tanaka Akira special")

  \g<name>    call by group name
  \g<n>       call by group number (n >= 1)

  * left-most recursive call is not allowed.
     ex. (?<name>a|\g<name>b)   => error
         (?<name>a|b\g<name>c)  => OK

  * Call by group number is forbidden if named group is defined in the pattern
    and ONIG_OPTION_CAPTURE_GROUP is not setted.

  * If the option status of called group is different from calling position
    then the group's option is effective.

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


10. Captured group

  Behavior of the no-named 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 captured group.

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

     (...) is treated as a no-captured group (?:...).

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

     (...) is treated as a no-captured group (?:...).
     numbered-backref/call is not allowed.

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

     (...) is treated as a captured 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)

  These options are not implemented in Ruby level.


-----------------------------
A-1. Syntax depend options

   + ONIG_SYNTAX_RUBY
     (?m): dot(.) match newline

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


A-2. Original extensions

   + named group     (?<name>...)
   + named backref   \k<name>
   + subexp call     \g<name>, \g<group-num>


A-3. Lacked features compare with perl 5.8.0

   + [:word:]
   + \N{name}
   + \l,\u,\L,\U, \X, \C
   + (?{code})
   + (??{code})
   + (?(condition)yes-pat|no-pat)

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

   * \p{property}, \P{property}
     This is effective on ONIG_SYNTAX_PERL and ONIG_SYNTAX_JAVA.
     Alnum, Alpha, Blank, Cntrl, Digit, Graph, Lower,
     Print, Punct, Space, Upper, XDigit, ASCII are supported.

     Prefix 'Is' of property name is allowed in ONIG_SYNTAX_PERL only.
     ex. \p{IsXDigit}.

     Negation operator of property is supported in ONIG_SYNTAX_PERL only.
     \p{^...}, \P{^...}


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

   + add look-behind
     (?<=fixed-char-length-pattern), (?<!fixed-char-length-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 incompleted left brace as an usual string.
     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 numbered character.
     ex. /\x61/i =~ "A"
   + In the range quantifier, the number of the minimum is omissible.
     /a{,n}/ == /a{0,n}/
     The simultanious abbreviation of the number of times of the minimum
     and the maximum is not allowed. (/a{,}/)
   + Zero-length match in infinite repeat stops the repeat,
     then changes of the capture group status are checked as stop condition.
     /(?:()|())*\1\2/ =~ ""
     /(?:\1a|())*/ =~ "a"


A-5. Disabled functions by 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 first byte as a character in UTF-8 is allowed.
     (which is the same as GNU regex of Ruby)

       /./u =~ "\xa3"

     Although it is possible to validate,
     it will become slower than now.


   + Character types of multibyte encoded characters are not correct.

     All multibyte encoded characters are treated as word(\w) character.
     (and POSIX bracket [:graph:], [:print:] type)

     In UTF-8 and UTF-16, only the multibyte character in code point
     [U+0080 - U+00FF] is correctly judged in a character type.

// END
