Changes
copy fixed comment from sandbox: this has been wrong for years, and when I corrected it years ago, it just got overwritten later, so rather tan see it happen again, I'm just making the change in the live module now
{{#invoke:String|match|source_string|pattern_string|start_index|match_number|plain_flag|nomatch_output}}
OR
{{#invoke:String|posmatch|s=source_string|pattern=pattern_string|start=start_index
|match=match_number|plain=plain_flag|nomatch=nomatch_output}}