Need regex help to replace links in a string

[eluser]ehicks727[/eluser]
I've got a string that I need to turn properly formatted urls into anchor text.

For example...

Code:

$string = "http://www.example.com"

turns into...

Code:

'<a href="http://www.example.com">http://www.example.com</a>'

I appreciate the help!! Thank you.