for [aXXbXXc]:
        [a]     undef
        [b]     undef
        undef   [c]
for [abcd]:
        undef   [a]
        undef   [b]
        undef   [c]
        undef   [d]
for [a little 'quote XX' quote stuff XX other]:
        [a little 'quote XX' quote stuff ]      undef
        undef   [ ]
        undef   [o]
        undef   [t]
        undef   [h]
        undef   [e]
        undef   [r]