RxCaseConverter.java

    private static String nonWordsReg = "[\\W\\_]+";
    private static String non = "";
    private static String spaceReg = "\\s";
    private static String space = " ";
    private static String upper = "(\\p{Ll})(\\p{Lu})";
    private static String upperGroup = "$1 $2";


    public Observable<String> camelCase(String source) {
        return lowerCase(source)
                .map(string -> replaceSpaceWithUpperCase(new StringBuilder(string)).toString())
                .map(string -> string.replaceAll(spaceReg, non));
    }

    public Observable<String> kebabCase(String source) {
        return Observable.just(source)
                .map(string -> string.replaceAll(upper, upperGroup))
                .map(string -> string.replaceAll(nonWordsReg, space))
                .map(String::trim)
                .map(string -> replaceSpaceWithLowerCase(new StringBuilder(string)).toString())
                .map(string -> string.replaceAll(spaceReg, "-"))
                .map(string -> string.substring(0, 1).toUpperCase().concat(string.substring(1)))
                .map(String::toLowerCase);
    }

    public Observable<String> snakeCase(String source) {
        return Observable.just(source)
                .map(string -> string.replaceAll(upper, upperGroup))
                .map(string -> string.replaceAll(nonWordsReg, space))
                .map(String::trim)
                .map(string -> replaceSpaceWithLowerCase(new StringBuilder(string)).toString())
                .map(string -> string.replaceAll(spaceReg, "_"))
                .map(string -> string.substring(0, 1).toUpperCase().concat(string.substring(1)))
                .map(String::toLowerCase);
    }

    public Observable<String> startCase(String source) {
        return Observable.just(source)
                .map(string -> string.replaceAll(upper, upperGroup))
                .map(string -> string.replaceAll(nonWordsReg, space))
                .map(String::trim)
                .map(string -> replaceSpaceWithUpperCase(new StringBuilder(string)).toString())
                .map(string -> string.substring(0, 1).toUpperCase() + string.substring(1));
    }


    private static StringBuilder replaceSpaceWithUpperCase(StringBuilder source) {
        for (int i = -1; (i = source.indexOf(space, i + 1)) != -1; i++) {
            source.setCharAt(i + 1, Character.toUpperCase(source.charAt(i + 1)));
        }
        return source;
    }

    private static StringBuilder replaceSpaceWithLowerCase(StringBuilder source) {
        for (int i = -1; (i = source.indexOf(space, i + 1)) != -1; i++) {
            source.setCharAt(i + 1, Character.toLowerCase(source.charAt(i + 1)));
        }
        return source;
    }