For the RegExps gurus!

I want to write a RegExp that converts something like this: $mem = " Hello world! http://www.helloworld.com\n Hello world! ftp://ftp.server.com\n Hello world www.helloworld.com" how can I write a single regexp that converts $mem to this: $mem = " Hello world! <a href="http://www.helloworld.com">http://www.helloworld.com</a>\n Hello world! <a href="ftp://ftp.server.com">ftp://ftp.server.com</a>\n Hello world <a href="http://www.helloworld.com">http://www.helloworld.com</a>" As you noticed that the second line didn't have the protocol specifier and it took by default "http://" and the ones that had a protocol specifier were kept as is.