(?-xism:^foo\.c$)
That is a perfectly valid regexp. The (?- allows one to specifiy regexp option inside the regexp pattern itself.
Actually, it's "(?OPT-NOPT:PAT)", including the colon, which is a variation on the old familiar theme of "(?:PAT)". The hyphen and following negated options are optional.

I'll give a somewhat more detailed example: 

  qr/^foo.*\.c$/si

[download]
as a string is 
  (?si-xm:^foo.*\.c$)

[download]
Thus: the enabled options come right after the question mark, disabled options come after that, between a hyphen and the colon.

These are the only four options that can actually be used in this manner. Thus: the list of options is complete.

In reply to Re: Re: Getting regexp from Regexp references by bart
in thread Getting regexp from Regexp references by steves

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