c   Do not reset search pos on a failed match when /g is in effect.
g   Match globally, i.e., find all occurrences.
i   Do case-insensitive pattern matching.
m   Treat string as multiple lines.
o   Compile pattern only once.
s   Treat string as single line.
x   Use extended regular expressions.
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If you don't want the position reset after failure to match, add the "//c", as in "/regexp/gc".

Because it's not a regex flag, per se. It's a pattern matching operator flag. perlre only covers /i, /m, /s, and /x.

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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

"man perlop" has the search options covered.

I am starting to think that it may not be hung but searching for a long long time (many 10s of minutes). A product of the cascaded non-greedy searches failing at the last pattern. I will run it and see if it ever finishes. The file I am parsing is pretty small (few thousand lines) so it should finish if it can finish.