Perl expression to extract URL from text?

I'm looking for a perl expression to extract urls from text. The more flexible it is the better.

For example, it would be nice if it could find all of these:
http://www.yahoo.com
yahoo.com
www.yahoo.com/mypage/

Thanks so much for the help! I will be using this in php if it makes any difference.

use Regexp::Common;
while (<>) {
while (m/$RE{URI}{-keep}/g) {
print "found a URI =$1=\n";
}
}