This might not apply in your specific case, but perhaps a nice idea anyway:

Suppose you know an exact substring that occurs before the match. Then you can find it's position with rindex, set pos to that value, and add \G.* in front of your regex.

In reply to Re: Optimizing a regex that replaces near the end of a string by moritz
in thread Optimizing a regex that replaces near the end of a string by perl5ever

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