+Arguments:
+
+ rx: the regex to match against
+ sv: the SV being matched: only used for utf8 flag; the string
+ itself is accessed via the pointers below. Note that on
+ something like an overloaded SV, SvPOK(sv) may be false
+ and the string pointers may point to something unrelated to
+ the SV itself.
+ strbeg: real beginning of string
+ strpos: the point in the string at which to begin matching
+ strend: pointer to the byte following the last char of the string
+ flags currently unused; set to 0
+ data: currently unused; set to NULL
+
+