? is special for the regexp machine. What you want is:

    /^(?:RE:\s*|FW:\s*)*\Q$match_string\E$/
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The \Q ... \E tells the regexp engine that anything in between should be taken "as is". I also removed the \i*. I do not know what you think \i is, but any string will end with zero or more of them. Finally, one uses ( ) or (?: ) for grouping, [RE:\s*|FW:\s*]* means something else. That would match any string consisting of whitespace, the capital letters E, F, R, W and the characters : and |.

Abigail