@fbongiovanni this sounds simple in theory, but it is extremely complex to do, that's why you couldn't find one already made on-line.
Of all the features in JSON Schema, the one that makes deriving an example from the hardest is regular expression validations. To be able to generate a string that matches a regEx, from the regEx itself, requires that you write a regEx engine yourself.
Most simple features can be easily implemented (character classes, negations, greediness, etc), but when you get to nevative-lookahead|behind, things get a bit harder.
This is a matter of investment/ROI, we can write something that emits valid examples that match 75% of regular expressions, whereas if the schema uses overly complex regexes, they fall out of the scope of the generator.