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I wrote this regex to match all href and src links in an HTML page; (I know I should be using a parser; this just experimenting):

/((href|src)\=\").*?\"/ # Without look-behind

It works fine, but when I try to modify the first portion of the expression as a look-behind pattern:

/(?<=(href|src)\=\").*?\"/ # With look-behind

It throws an error stating 'invalid look-behind pattern'. Any ideas, whats going wrong with the look-behind?

Jikku Jose
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1 Answers1

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Lookbehind has restrictions:

   (?<=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.

You cannot put alternatives in a non-top level within a (negative) lookbehind.

Put them at the top level. You also don't need to escape some characters that you did.

/(?<=href="|src=").*?"/
lulalala
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sawa
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  • Oh thanks! Didn't know such a rule! Thanks for the Oniguruma cheat sheet too! Though, not very relevant to the concept: the above can be solved by having the negative look behind modified like this, hence having the character length same: `/(?<=(?:ref|src)\=\").*?\"/` – Jikku Jose Nov 13 '13 at 07:36