Well, it's clear to see why people are lead astray. In this case, RFC 3986, which defines what a URI actually is, itself actually proposes a regex[0]. It even

Well, it's clear to see why people are lead astray. In this case, RFC 3986, which defines what a URI actually is, itself actually proposes a regex[0]. It even claims, and I quote, 'the "first-match-wins" algorithm is identical to the "greedy" disambiguation method used by POSIX regular expressions'. Of course, it doesn't actually validate anything... it just performs rudimentary field extraction. Trying to do anything beyond that with regex is madness. The ABNF grammar is fairly simple though and, I think, free of ambiguities. I've had luck converting it straight in to PEG form. It's not a trivial or wholly useful endeavour though, and, if you do so, remember to check the errata. For HTTP you'll also have to add in the changes from RFC 7230[1]. Oh, and of course, none of this validates DNS names, their labels, etc. for length, the "LDH rule", or the public suffix list[2], or IP addresses to check whether they have publicly routable prefixes. Bottom line is, if you want to validate a URL, the best thing to do, much like e-mail, is to just try and GET it. [0] https://tools.ietf.org/html/rfc3986#appendix-B https://tools.ietf.org/html/rfc3986#appendix-B [1] https://tools.ietf.org/html/rfc7230#section-2.7.1 https://tools.ietf.org/html/rfc7230#section-2.7.1 [2] https://publicsuffix.org/ https://publicsuffix.org/