With RegeXPath, you can quickly test regular expressions and review the matches and their captures. As matches are found, they are added to the result tree so that you can easily navigate through the results.
It's also extremely easy to test XPath queries. When you evaluate an XPath expression, the results are placed in the Output tab for you to view.
Some help is provided for the elements of regular expressions and XPath queries. The Insert menu inserts expression constructs into the Expression window.
Of course there are other features, such as Captures, Bookmarks and syntax highlighting, but you should really try the demo to see those in action. Download it here.