You have not yet indicated what part of (A|B|C|D) does not meet your needs. If I understand your question clearly, you've asked us to translate the expression
[ABCD] /
(A|B|C|D) into some other format used by this other language neither of which you have told us how to write. Are we to guess that
http://www.xrce.xerox.com/competencies/content-analysis/fsCompiler/fssyntax.html#Complex is the language you wish to write your expression in? If so, it appears tthat Xerox used square brackets in the place of perl's parentheses. This leaves me with the probable expression
[ A | B | C | D ].