Matcher, Pattern, MatchResults, streaming.

MatchResultsStream.java

 /*
        * Compiles a regex pattern, calls a Matcher on with the input string as a param,
        * calls .results() which returns a stream of MatchResults for each successful match.
        * .count() counts the elements in the stream.
        * */
        String input = new Scanner(System.in).nextLine(); // "accggaabb"
        System.out.println((double) Pattern.compile("(?:[cg])").matcher(input).results().count() / input.length() * 100);
        // 44.44444444444444