Not sure what you mean.. Should the complete text be only text, mixed text/url or url only ?
anyway, have a look at pear.php.net/validate
If you would would like hyperlinks within the text, you could do some string replace after a complex regex..
Easiest would be to have more field, one for each type (url/email/name/comment). This is much more flexible and you could still mix the text after input.