

<P>A back reference matches whatever actually matched the capturing subpattern in the current subject string, rather than anything matching the subpattern itself. So the pattern</P>

<P>(sens|respons)e and \1ibility</P>

<P>matches "sense and sensibility" and "response and responsibility", but not "sense and responsibility". If caseful matching is in force at the time of the back reference, then the case of letters is relevant. For example,</P>

<P>((?i)rah)\s+\1</P>

<P>matches "rah rah" and "R