Syntax.txt

Path: Syntax.txt
Last Update: Mon Jul 16 10:05:51 +0200 2007

RUBY REGULAR EXPRESSION SYNTAX

Syntax Elements

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

Characters

\t
horizontal tab (0x09)
\v
vertical tab (0x0B)
\n
newline (0x0A)
\r
return (0x0D)
\b
back space (0x08)

\b is effective in character class […] only

\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)

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:

  • 0009, 000A, 000B, 000C, 000D, 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 hexadecimal digit char

Character Properties

 \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:

  • Hiragana, Katakana

Works on UTF8, UTF16, UTF32:

  • Any, Assigned, C, Cc, Cf, Cn, Co, Cs, L, Ll, Lm, Lo, Lt, Lu, M, Mc, Me, Mn, N, Nd, Nl, No, P, Pc, Pd, Pe, Pf, Pi, Po, Ps, S, Sc, Sk, Sm, So, Z, Zl, Zp, Zs, Arabic, Armenian, Bengali, Bopomofo, Braille, Buginese, Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic, Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin, Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi

Quantifiers

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}?)

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)

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
matching start position

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 ^)

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

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

Not Unicode Case:

alnum
alphabet or digit char
alpha
alphabet
ascii
code value: [0 - 127]
blank
\t, \x20
cntrl
control
digit
0-9
graph
include all of multibyte encoded characters
lower
lower case
print
include all of multibyte encoded characters
punct
punctuation
space
\t, \n, \v, \f, \r, \x20
upper
upper case
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:]]
punct
Connector_Punctuation | Dash_Punctuation | Close_Punctuation | Final_Punctuation | Initial_Punctuation | Other_Punctuation | Open_Punctuation
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

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.)

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.

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.

Back reference with nest level

\k<name+n>
n: 0, 1, 2, …
\k<name-n>
n: 0, 1, 2, …

Destinate relative nest level from back reference position.

Examples: 

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

   r = ORegexp.compile(<<'__REGEXP__'.strip, :options => Oniguruma::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

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.

    Example: 

      (?<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 Oniguruma::OPTION_CAPTURE_GROUP is not set.
  • If the option status of called group is different from calling position then the group‘s option is effective.

    Example: 

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

Captured group

Behavior of the no-named group (…) changes with the following conditions. (But named group is not changed.)

case 1
ORegexp.new( ’…’ ) (named group is not used, no option)

… is treated as a captured group.

case 2
ORegexp.new( ’…’, :options => OPTION_DONT_CAPTURE_GROUP ) (named group is not used, ‘g’ option)

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

case 3
ORegexp.new( ’…(?<name>…)…’ ) (named group is used, no option)

(?<name>…) is treated as a no-captured group (?:…)

numbered-backref/call is not allowed.

case 2
ORegexp.new( ’…’, :options => OPTION_CAPTURE_GROUP ) (named group is used, ‘G’ option)

(?<name>…) is treated as a captured group (?:…)

numbered-backref/call is allowed.

where

  • g: OPTION_DONT_CAPTURE_GROUP
  • G: OPTION_CAPTURE_GROUP

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

Syntax dependent 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

Original extensions

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

Lacking features compare with perl 5.8.0

  • \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.

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-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. <code>/\x61/i =~ "A"<code>
  • In the range quantifier, the number of the minimum is omissible.

    <code>/a{,n}/ == /a{0,n}/<code>

    The simultanious abbreviation of the number of times of the minimum and the maximum is not allowed. (/a{,}/)

  • <code>a{n}?<code> is not a non-greedy operator. <code>/a{n}?/ == /(?:a{n})?/<code>
  • invalid back reference is checked and cause error. /\1/, /(a)\2/
  • 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"

Problems

  • Invalid encoding byte sequence is not checked in UTF-8.
  • Invalid first byte is treated as a character. 
     /./u =~ "\xa3"
  • Incomplete byte sequence is not checked. 
     /\w+/ =~ "a\xf3\x8ec"

[Validate]