For the first question, use:

  /ABC(?:DD.|D.D|.DD).E/;  # Add /s if newline is acceptable near the 
+Ds
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As for the generic case, you don't. Regexes aren't suitable for that. For particular cases, you may be able to get away with listing all possibilities, or by using (?{...}), but in a general case, you'd be using the wrong tool.