RegexSR is a Java-based creator and tester of regular expressions (also known as regex). With this free tool you will not need to run your code to make sure that the Java regular expression you want to use does work. It comes with a useful repository where you can store all your newly-created regular expressions.
The interface provided is split into two panels - an upper panel for you to write the text where the regex will be tested on, and a lower panel to show you the testing results. They are both synchronized, and the results are shown in real time. However, before making use of these two panels, you will need to enter the regular expression in question in the provided "Search" text box. Any mistake found in the expression itself will be shown to you as you type it, making the whole testing process extremely fast and reliable. When using a regular expression to replace one character string by another, you can also type in the replace value in the corresponding box, and the change proposed will be performed in all the instances found, and also in real time. Once fully tested, you are free to send the generated regular expression to the repository.
As a bonus, this free utility comes with an extremely useful summary of regular expressions. In this tab you will find an extensive list of constructs and matches to help you build your own regular expressions.