an overall match they can also produce sub-expression
        matches: each sub-expression being delimited in the pattern by a pair of
        parenthesis (...). There has to be some method for reporting sub-expression
        matches back to the user: this is achieved this by defining a class <code class="computeroutput"><span class="identifier">match_results</span></code> that acts as an indexed collection
        of sub-expression matches, each sub-expression match being contained in an
        object of