We aren’t sure how we feel about [pemistahl’s] grex program. On the one hand, we applaud a program that can take some input samples and produce a regular expression. On the other hand, it might be just as hard to gather example data that produces the correct regular expression. Still, it is an interesting piece of code.
Even the author suggests not to use this as an excuse to not learn regular expressions, since you’ll need to check the program’s output. It is certain that the results will match your test cases, but it isn’t certain that it won’t accept things you didn’t expect. Bad regular expressions have been the source of some deeply buried bugs.