how to negate any regular expression in Java

I know this is a really old question but hopefully my answer can help anyone looking for this in the future.

While Alan Moore's answer is almost correct. You would need to group the whole regex too, or else you risk anchoring only part of the original regex.

For example if you want to negate the following regex: abc|def (which matches either "abc" or "def"

Prepending (?! and appending $).*. You will end up with (?!abc|def$).*.

The anchor here is only applying to def, meaning that "abcx" will not match when it should.

I would rather prepend (?!(?:and append )$).*.

String negateRegex(String regex) {
    return "(?!(?:" + regex + ")$).*";
}

From my testing it looks like negateRegex(negateRegex(regex)) would indeed be functionally the same as regex.

You need to add anchors. The original regex (minus the unneeded parentheses):

/.{0,4}

...matches a string that contains a slash followed by zero to four more characters. But, because you're using the matches() method it's automatically anchored, as if it were really:

^/.{0,4}$

To achieve the inverse of that, you can't rely on automatic anchoring; you have to make at least the end anchor explicit within the lookahead. You also have to "pad" the regex with a .* because matches() requires the regex to consume the whole string:

(?!/.{0,4}$).*

But I recommend that you explicitly anchor the whole regex, like so:

^(?!/.{0,4}$).*$

It does no harm, and it makes your intention perfectly clear, especially to people who learned regexes from other flavors like Perl or JavaScript. The automatic anchoring of the matches() method is highly unusual.