I found a very nice URL regex matcher on this site: http://daringfireball.net/2010/07/improved_regex_for_matching_urls . It states that it's free to use and that it's cross language compatible (including Javascript). First of all, I have to escape some of the slashes to get it to compile at all. When I do that, it works fine on Rubular.com (where I generally test regexes), with the strange side effect that each match has 5 fields: 1 is the url, and the extra 4 are empty. When I put this in JS, I get the error "Invalid Group". I am using Node.js if that makes any difference, but I wish I could understand that error. I'd like to cut back on the unnecessary empty match fields, but I don't even know where to begin diagnosing this beast. This is what I had after escaping:
(?xi)\b((?:[a-z][\w-]+:(?:\/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}\/)(?:[^\s()<>]+|\(([^\s()<>]+|(\([^\s()<>]+\)))*\))+(?:\(([^\s()<>]+|(\([^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’] ))
Actually, you don't need the first capturing group either; it's the same as the whole match in this case, and that can always be accessed via $&. You can change all the capturing groups to non-capturing by adding ?: after the opening parens:
/\b(?:(?:[a-z][\w-]+:(?:\/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}\/)(?:[^\s()<>]+|\((?:[^\s()<>]+|(\(?:[^\s()<>]+\)))*\))+(?:\((?:[^\s()<>]+|(?:\(?:[^\s()<>]+\)))*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))/i
That "invalid group" error is due to the inline modifiers (i.e., (?xi)) which, as @kirilloid observed, are not supported in JavaScript. Jon Gruber (the regex's author) was mistaken about that, as he was about JS supporting free-spacing mode.
Just FYI, the reason you had to escape the slashes is because you were using regex-literal notation, the most common form of which uses the forward-slash as the regex delimiter. In other words, it's the language (Ruby or JavaScript) that requires you to escape that particular character, not the regex. Some languages let you choose different regex delimiters, while others don't support regex literals at all.
But these are all language issues, not regex issues; the regex itself appears to work as advertised.