use re 'eval';
    no strict 'refs';
    if (/(A)?(C)?(C)?(A)?(A)?(C)?([ACGTacgtNn]{6})?(CTA[ACGTacgtNn]{1}
+)?
         (A)?(T)?(G)?([ACGTacgtNn]{1,2})?(G)?(A)?(T)?(G)?(T)?(T)?
         (?(?{17 <= grep {defined $$_} 1 .. 19})|(?!))/x) {
        ...
    }
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Abigail