Fix crash with bulk pattern replacements (introduced with c83a93e)

"regex_match_text" and "regex_matches" being globals, performing
several searches and then the replacements separately lead to them
having unexpected values, resulting in incorrect behavior and crash.

Fix this by removing the globals and instead make the search functions
return match details.  Not only this fixes the issue, but also make the
code a lot more maintainable by not having globals introducing side
effects (proof of them being an issue is that c83a93e inadvertently
broke things bad).
Don't try to set std_out and std_err if they are NULL