How to generate random strings that match a given regexp?

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Random string that matches a regexp

No, it isn\'t. I\'m looking for an easy and universal method, one that I could actu

Answer 1

One rather ugly solution that may or may not be practical is to leverage an existing regex diagnostics option. Some regex libraries have the ability to figure out where the regex failed to match. In this case, you could use what is in effect a form of brute force, but using one character at a time and trying to get longer (and further-matching) strings until you got a full match. This is a very ugly solution. However, unlike a standard brute force solution, it failure on a string like ab will also tell you whether there exists a string ab.* which will match (if not, stop and try ac. If so, try a longer string). This is probably not feasible with all regex libraries.

On the bright side, this kind of solution is probably pretty cool from a teaching perspective. In practice it's probably similar in effect to a dfa solution, but without the requirement to think about dfas.

Note that you won't want to use random strings with this technique. However, you can use random characters to start with if you keep track of what you've tested in a tree, so the effect is the same.