            the whole match. Returns a null string if the index is invalid
                    or if the specified sub-expression did not participate in a match.
                  </p>
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                  <p>
                    <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="keyword">operator</span><span class="special">[](</span><span class="keyword">int</span>
           