(?-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.