my $pattern = join "|", map quotemeta, keys %replacements;

I think it would be good to also sort on length, as I discussed in Building Regex Alternations Dynamically. Although huck does make a good point that preserving the original ordering may be important too. Update after your reply: Good point about replacements within the replacements, whether the OP wants that or not and which method is more appropriate will depend on their requirements.

In reply to Re^2: string replacement by haukex
in thread string replacement by colox

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