GGGGGGGGAACCCCCCCCCCTTGGGGGGGGGGGGGGGGAACCCCCCCCCCTTGGGGGGGG

:- use_module(library(pio)).
:- use_module(library(dcg/basics)).
:- portray_text(true).

process(Xs) :- phrase_from_file(find(Xs), 'string.txt').

anyseq([]) -->[].
anyseq([E|Es]) --> [E], anyseq(Es).

begin --> "AA".
end -->"TT".

find(Seq) -->
     anyseq(_),begin,anyseq(Seq),end, anyseq(_).

?- process(Xs).
 Xs = "CCCCCCCCCC" ;
 Xs = "CCCCCCCCCCTTGGGGGGGGGGGGG...CCCCC" ;
 Xs = "CCCCCCCCCC" ;
false.