This is just an idea, but it should work.
You need to check all the regular expressions from steps in a result report.
1) create a complete report running all active scenarios with
2) save all the regular expressions using running behat with
Now you have 2 files all scenarios and all regular expressions, now all you should need to do is to do a
foreach in the regular expressions and look it in the report file and maybe count the matches as well.
The search could be done in php or with any other tool that can handle regular expressions.