You're right. It looked like it worked on the original to me because it matched "cat" against "" which "found" it. The + is a way to force constants to evaluate, also works, in some contexts, with spelling it out as the sub it is, RX(). Doesn't work here though. :(

In reply to Re^4: calling subroutines within a regular expression? by Your Mother
in thread calling subroutines within a regular expression? by Anonymous Monk

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