This just came up tonight and I hate to continue to pick on the validate_url function, but someone wanted to link the following wikipedia page ( which is a valid URL ;) )
http://en.wikipedia.org/wiki/Sprite_(computer_graphics)
and the current function says that it's an invalid URL (REGEX doesn't allow the parentheses).
A work-a-round, until the bug is fixed, is to uncomment the url search/replace lines in the BB Code plugin and use
[url=http://en.wikipedia.org/wiki/Sprite_(computer_graphics)]text you want[/url]
(Work-a-round works in both DAWN and v1.6 alpha versions).
[url=http://forums.b2evolution.net/viewtopic.php?t=7181]Original Thread[/url]
Hope this helps.