Comment 6 for bug 186578

Will Drewry reported a flaw in the way libicu processes certain regular
expressions. He reports:

    On regular expression compilation, illegal backreferences may refer to the
    non-existent capture group '0'. When these are builts, they will result
    in corrupt REStackFrames which will be used at a later point. Crashes may
    result in out of band reads or writes depending on the regular expression
    being executed.