Is it possible to generate an example string based on a regex pattern?

In my application the user can enter his own regex pattern into a text box so he can force a certain input for another user/text box. Is it possible for the user to see an example of a string that would match the regex he has entered? For example if he was to enter: ^[A-Z]{2}$, it would generate a string l开发者_如何学Goike "XX" to show the user he can only enter two capital letters.

If there's no easy way to accomplish this (and I assume there isn't), how difficult would it be to build? Or does something like this already exist?

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Check out Xeger. It looks like it can do what you want. It's in Java though.

Here is an example from the test suite:

   @Test
    public void shouldGenerateTextCorrectly() {
        String regex = "[ab]{4,6}c";
        Xeger generator = new Xeger(regex);
        for (int i = 0; i < 100; i++) {
            String text = generator.generate();
            assertTrue(text.matches(regex));
        }
    }

Update: thanks to Nikos Baxevanis, the dk.brics.automaton have been ported to C# at https://github.com/moodmosaic/Fare

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For C# you may also want to look at project Fare. For more details have a look at this answer.

Example

var regex = @"((mailto\:|(news|(ht|f)tp(s?))\://){1}\S+)";
var xeger = new Xeger(regex);

var result = Regex.IsMatch(xeger.Generate(), regex);
// -> Prints 'true'

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I once needed such thing too, so I created a simple program with gui using xeger lib mentioned above. Simply run .jar from dist folder (jre is required) https://github.com/ogyct/SampleFromRegex