An all-in-one Regex Evaluator using the Java standard API (java.util.regex).
You can find the Plugin in Window -> Regex Tester
It gives live feedback to changes on the regular expression, replacement and input. Errors in the regular expression are documented and you are able to set all flags provided by the standard API (e.g. CASE_INSENSITIVE, MULTILINE...).
Plugin available in Netbeans Plugin Center: http://plugins.netbeans.org/plugin/63405