How to create a negated charclass pattern?

I need a method taking a patte开发者_StackOverflowrn as an argument and returning the negated pattern. The input pattern is always a charclass (otherwise it'd impossible). The straightforward attempt

public Pattern negate(Pattern p) {
    return Pattern.compile("[^" + p.pattern() + "]");
}

private void test() {
    final Pattern p = Pattern.compile("[a-z]");
    final Pattern n = negate(p);
    System.out.println(n.matcher("0").matches());
}

fails because of [ in [^[ (unlike in [^xyz[) being taken literally instead of as a start of a nested charclass.

You can't nest character classes like that.

If you always know that you're getting a character class, the easiest way would be to trim the input string of its brackets:

public Pattern negate(Pattern p) {
    return Pattern.compile("[^" + p.pattern().substring(1));
}

A more robust method would be to parse the input with regular expresions... :)