What do you think of my regex for URL validation?
Your regex has many false positives (matches without a valid URL being there), and many false negativs (URLs you don't recognize). You should read the appropriate RFCs which contain relevant parts of the grammar.

Here are some failure scenarios:


⊖ You don't match schemes other than HTTP or HTTPS. Consider FTP, SFTP, data URLs, and other widespread schemes like file:, mailto:, irc: or skype:. Consider also that the scheme is sometimes omitted, so that the same protocol as the current document is assumed. Test cases:

ftp://example.com/
data:text/html,&lt;p&gt;Hello World!
mailto:foo@example.com
//example.com/


Every scheme has a slightly different syntax, so you'll have to decide which ones you support. Note that matching a HTTP(S) URL is different from matching all URLs. 
⊕ You match some domain names that are syntactically correct, but semantically only consist of a top level domain. You may or may not wish to rule these out:

http://co.uk/


See the Mozilla Public Suffix List for the only reliable way to do this.
⊖ Some host names don't have a TLD:

http://localhost/

⊕ You match loads of stuff that isn't ok, e.g.:

999.999.999.999/this-is-no-ip4-address
-.-

⊖ You don't have support for ports:

http://example.com:80/

⊖ You don't have support for IPv6 IPs.

http://::1/
http://[::1]:80/

⊖ You can't match any authority in the host part:

ftp://user:password@example.com/

⊖ You don't support URL-encoding:

http://example.com/some%20path

⊖ You don't support Unicode chars:

http://example.com/smørrebrød/greek-Λεττερς


Consider also that the host name may contain Unicode characters, but will have an equivalent punycode representation.
⊖ Coming to think of it, you actually forbid quite a lot of (special) characters, although they would be perfectly valid (the host part is more restricted).

http://example.com/stuff_(in parens)/

⊖ In a URL, the path is followed by a single query string, introduced by a ?. Note that the semicolon ; is the more modern, but equivalent alternative to the &amp; separator. The + means a literal space here. The query string may only be followed by a fragment, which is separated by #. Other instances have to be encoded.




For an overview of URL syntax, start with this Wikipedia article. Also look at URI Schemes. These articles also link to relevant specs.