I came across your tool, and while it appears interesting, the first thing I tried it with appears to have crashed it somehow.
I was trying to run a SpamAssassin rule against a message from the SpamAssassin public corpus. Both of these are publicly available from http://www.spamassassin.org but for completeness, I am also attaching them below in exactly the form I was using them.
Running the regex from 20_head_tests_cf_UNRESOLVED_TEMPLATE.txt with the /m modifier against the message headers in easy_ham_00001-head.txt and clicking on the "Tree" tab causes a lot of output to be printed on standard output (cf. third attachment) and the main window to disappear after a short while. The process is still alive, consuming around 90-95% of the CPU, though not producing a lot of load (load average still well below one). I finally killed it after allowing it to run for some 10 minutes wall clock time. Incidentally, ctrl-C causes a message "Quittingregex-coach/regex-coach : Interrupted - quitting" to be printed (on standard output, not standard error, sigh), but does not kill the process.
PS. Would you mind if your list was signed up for Gmane http://www.gmane.org/? If I go ahead and sign up the list, do you have an mbox archive handy which could be imported into Gmane?
/* era */