Before writing a UI that accepts regular expressions, you should review the (?{code}) and (??{code}) patterns in perlre. You will want to remember this curious statement...

use re 'eval';  # Open Pandora's box
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and make sure it is NOT being used*... otherwise your users may find some remarkably interesting patterns to type in.

 

* (at least, not in the same section of code, and not without turning on taint.)