From: Simon Strandgaard <neoneye@...>
Date: 2004-09-12T20:32:11+09:00
Subject: Re: look-behind in oniguruma

On Sunday 12 September 2004 13:07, Simon Strandgaard wrote:
> On Sunday 12 September 2004 06:54, Andrew Johnson wrote:
> > On 12 Sep 2004 03:44:24 GMT, Phil Tomson <ptkwt@aracnet.com> wrote:
> > > Apparently oniguruma supports look-behind.  Is there any documentation
> > > on how to use this feature?
> >
> > Essentially, they are the same as look-aheads ... zero-width assertions,
> > except that the look-behind expression must be a fixed width pattern (no
> > indeterminate quantifiers), and no captures are allowed in a negative
> > look-behind
>
> Oniguruma supports alternation inside lookbehind, so you can get a similar
> behavior as quantifiers.
>
> AEditor's regexp engine supports variable width lookbehind, where you
> can use quantifiers inside lookbehind.. (with inversed left-most-longest
> rule).
>
> It would be good if Oniguruma had support for quantifiers inside
> lookbehind.


(here is an example with infinite quantifiers)

irb(main):016:0> re = NewRegexp.new('(?<!(ab)+|(cd){2,}).')
=> +-Sequence
  +-Lookbehind negative
  | +-Alternation
  |   +-Repeat greedy{1,-1}
  |   | +-Group capture=1
  |   |   +-Sequence
  |   |     +-Inside set="a"
  |   |     +-Inside set="b"
  |   +-Repeat greedy{2,-1}
  |     +-Group capture=2
  |       +-Sequence
  |         +-Inside set="c"
  |         +-Inside set="d"
  +-Outside set=U-000A
irb(main):017:0> 'qwerty'.gsub5(re, 'Z')
=> "ZZZZZZ"
irb(main):018:0> 'qweabrty'.gsub5(re, 'Z')
=> "ZZZZZrZZ"
irb(main):019:0> 'cdcdqwerty'.gsub5(re, 'Z')
=> "ZZZZqZZZZZ"
irb(main):020:0> 'cdqwerty'.gsub5(re, 'Z')
=> "ZZZZZZZZ"
irb(main):021:0>

--
Simon Strandgaard