in reply to using a regex which is in a string

You can use Safe to safely evaluate the substitution. See Re: Want to get regexp list from file.
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Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart