      Returns the length of what matched sub-expression i. If <code class="computeroutput"><span class="identifier">i</span> <span class="special">=</span>
                    <span class="number">0</span></code> then returns the length
                    of the whole match. Returns <code class="computeroutput"><span class="identifier">RegEx</span><span class="special">::</span><span class="identifier">npos</span></code>
                    if the supplied index is invalid, or if the specified sub-expressi