I fail to see the problem. What part of the problem involving writing [ABCD] as (A|B|C|D) escapes you?

It doesn't escape. I am gonna used that in a different sytem whose regular expressions are defined differently. I am trying to make the translation between that system and the user by perl.

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 ].