Replace buggy Matcher.replaceAll in Android with a working version. Does not replace uncaptured group references with 'null'

replaceAll.java

// Implementation of Matcher.replaceAll differs in Android and JDK
// In JDK, if a group has not been captured, it is replaced with ""
// In android, it is replaced with the string 'null'
// This is annoying. Can't really be fixed without fully encapsulating the class
// So, rewriting replaceAll algorithm
// So we'll just do backref replacement and pass it back to the original replaceAll
private String replaceAll(Matcher matcher, String input, String replacement) {
    // Process substitution string to replace group references with groups
    int cursor = 0;
    StringBuilder result = new StringBuilder();

    while (cursor < replacement.length()) {
        char nextChar = replacement.charAt(cursor);
        if (nextChar == '\\') {
            cursor++;
            nextChar = replacement.charAt(cursor);
            result.append(nextChar);
            cursor++;
        } else if (nextChar == '$') {
            // Skip past $
            cursor++;
            // The first number is always a group
            int refNum = (int)replacement.charAt(cursor) - '0';
            if ((refNum < 0)||(refNum > 9))
                throw new IllegalArgumentException(
                        "Illegal group reference");
            cursor++;

            // Capture the largest legal group string
            boolean done = false;
            while (!done) {
                if (cursor >= replacement.length()) {
                    break;
                }
                int nextDigit = replacement.charAt(cursor) - '0';
                if ((nextDigit < 0)||(nextDigit > 9)) { // not a number
                    break;
                }
                int newRefNum = (refNum * 10) + nextDigit;
                if (matcher.groupCount() < newRefNum) {
                    done = true;
                } else {
                    refNum = newRefNum;
                    cursor++;
                }
            }
            // Append group
            if (matcher.group(refNum) != null)
                result.append(matcher.group(refNum));
        } else {
            result.append(nextChar);
            cursor++;
        }
    }

    return matcher.replaceAll(result.toString());
}