Problem with quantifiers and look-behind

Question

### Ruby 1.8.7 ###

require \'rubygems\'
require \'oniguruma\' # for look-behind

Oniguruma::ORegexp.new(\'h(?=\\w*)\')
# => /h(?=\\w*)/

Oniguruma::ORegexp.new(\         


        

Answer 1

I was banging my head against the same problem, and Borealid's answer helped explain the issue well.

However, that got me thinking. Maybe the quantifier does not need to be inside the lookbehind, but can be applied on the lookbehind itself?

"hello".match(/(?<=\w*)o/)
# => SyntaxError: (irb):3: invalid pattern in look-behind: /(?<=\w*)o/

"hello".match(/(?<=\w)*o/)
# => #<MatchData "o">

So now we have a variable number of constant-length lookbehinds. Seems to bypass the issue for me. :)

Answer 2

The issue is that Ruby doesn't support variable-length lookbehinds. Quantifiers aren't out per se, but they can't cause the length of the lookbehind to be nondeterministic.

Perl has the same restriction, as does just about every major language featuring regexes.

Try using the straightforward match (\w*)\W*?o instead of the lookbehind.