Looks fine to me. I don't know what Pel5Matcher is.
java.util.regex.Matcher is not thread-safe, so requires the synchronized
block.

You can write the method body more simple as:

        synchronized (MATCHER) {
            return MATCHER.matches(field, regex);
        }

If it is a frequently used method on a multiprocessor machine (even
desktops have multiple hardware threads these days), it may be worth
having a separate matcher for each thread:

    private static final ThreadLocal<Perl5Matcher> MATCHER =
        new ThreadLocal<Perl5Matcher>() {
            @Override
            protected Perl5Matcher initialValue() {
                return new Perl5Matcher();
            }
       };

    private static boolean validateRegex(
        final String field, final Pattern regex
    ) {
        return MATCHER.get().matches(field, regex);
    }

(Usual warning that you wont get class unloading with this sort of use
of ThreadLocal: http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6254531)

Tom Hawtin