While there are many good online and offline tools for testing regular expressions, I have yet to find a tool (besides RegexBuddy) that lets me see the steps that the regular expression engine takes to find a match (or to find that there is no match, for that matter.)

Does anyone know of any tools that do this? The reason I have initially excluded RegexBuddy is because it's commercial and I would prefer to look over my options before I decide to pay for it.

To make it clearer what I want, here is a screenshot from RegexBuddy (from their home page): http://img166.imageshack.us/img166/1272/debug.png

regex debugging
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