$pattern_A = "123";
$pattern_b = "456";

$string_1 = "1234567890";
$string_2 = "1236547890";

# Then apply regex/lookaround to search for adjacent pattern

if($adjacent_pattern == false){
     remove_pattern_and_six_following_characters;
     print("string_*");
}
else{
     do_something_else;
     print("string_*");
}