matchlen may be wrong

`int match_len;`
`char* s = "aa";`
`char* p = ".*a.*a";`
`int match_idx = re_match(p, s, &match_len);`

expected: match_len == 2
result: match_len == 3

Start by reproducing the supplied C example through re_match, then trace how that entry point calculates match_len for `.*a.*a`. Done means the example reports match_len == 2 rather than 3, with the relevant existing behavior still covered by tests or a focused regression case.

Written by the indexing model from the issue text.