Let's say that we have a regex and an index i , if we suppose that the set of string that match our regex are sorted in a lexicographical order, how could we get the i element of this list ? Edit : I added this simple example for more explanation : input : regex="[a-c]{1,2}"; index=4; in this case the ordered list of string that matches this regex contains those elements : a aa ab ac b ba bb bc c ca cb cc output : the 4th element which is ac ps: it's known that the string list that match the regex have infinite element, this doesn't have an impact on the proccess of extracting the element in the given index.