AH RAH", but not "RAH rah", even though the original capturing subpattern is matched caselessly.</P>

<P>There may be more than one back reference to the same subpattern. If a subpattern has not actually been used in a particular match, then any back references to it always fail. For example, the pattern</P>

<P>(a|(bc))\2</P>

<P>always fails if it starts to match "a" rather than "bc". Because there may be up to 99 back references, all digits following the backslash are taken as part of a potential b