s="identifier">sub_match</span></code></a> object that represents
        what matched a particular sub-expression within the regular expression. When
        class <a class="link" href="regex_token_iterator.html" title="regex_token_iterator"><code class="computeroutput"><span class="identifier">regex_token_iterator</span></code></a> is used to
        enumerate a single sub-expression with index -1, then the iterator performs
        field splitting: that is to say it enumerates one character sequence for
