I think you've got the right idea using 1 while s///, because you're matching from inside out rather than left to right. Here's one way to do the whole substitution all at once: 1 while s/\(([^\(\)]+)\)((?:\d+)?)/ $1 x ($2 || 1) /ge; This matches a parenthesized substring that does not itself contain any parenthesizes, and optionally a subsequent number, and replaces it with the substring, minus the parentheses, repeated the appropriate number of times.

In reply to Re: Regexps for Parsing Brackets in Chemical Formulae by chipmunk
in thread Regexps for Parsing Brackets in Chemical Formulae by Elgon

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