(?=\d) is a lookahead assertion.

In order to check for the presence of a digit before the rest of the match, you need a lookbehind assertion: (?<=\d)

See perlreref#EXTENDED-CONSTRUCTS for an overview of the different look-around constructs.

In reply to Re: zero-width assertions for extended regular expressions by smls
in thread zero-width assertions for extended regular expressions by johnrcomeau

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