It usually helps to rephrase what you are looking for. In your case, if I've understood your clarification elsewhere in this thread, you want to capture "all of the non-X characters at the end of the string." In code, that's /([^X]*)$/ and here it is in action:

$ perl -le '$_ = "abcX123"; print $1 if /([^X]*)$/;'
123
$ perl -le '$_ = "abc123"; print $1 if /([^X]*)$/;'
abc123
[download]

-sauoq
"My two cents aren't worth a dime.";