Convenience macro to get the REGEXP from a SV. This is approximately equivalent to the following snippet:
if (SvMAGICAL(sv)) mg_get(sv); if (SvROK(sv)) sv = MUTABLE_SV(SvRV(sv)); if (SvTYPE(sv) == SVt_REGEXP) return (REGEXP*) sv;
NULL will be returned if a REGEXP* is not found.
Returns a boolean indicating whether the SV (or the one it references) is a REGEXP.
If you want to do something with the REGEXP* later use SvRX instead and check for NULL.