So the specification changed? It seems that the rules for what you would like to match have expanded since your original post. Are you trying to match to capture? Or are you trying to match to reject strings that don't fit? I repeat, what are you trying to accomplish. What is it that you're doing; the bigger picture. It may be clear to you, but it's still quite vague to us.

A pattern that matches all of the above could be:

m/\[SYN(?:, (?:ACK|FIN|RST))?\]/
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There's an accurate answer to your question, as asked. Do you now care to elaborate so that our answers can be a little more specific, or have we answered your question satisfactorily?