Why don't you just get rid of the stuff in front of the www.foo.com stuff? I.e., assume its "bad" and put "http://" in front of it? Or for that matter just leave the http:// off once you've done step (1).

#!/usr/bin/perl -w
use strict;

my @urls = ('tp://www.foo.com/' , '://www.foo.com', 
            'http//:www.foo.com', 'www.foo.com'); 

foreach (@urls)
{
   s/^.*?www/www/;
   print "http://$_\n";
}

__END__
prints:
http://www.foo.com/
http://www.foo.com
http://www.foo.com
http://www.foo.com
[download]

Update: well, this could be more complex as a valid URL does not have to start with www, it could be xyz.tv, then I guess you would want: http://xyz.tv? It helps if you present a representative set of test cases.

It also helps if you can say something about the context of the application. Here I suppose you are trying to "guess" the user's intention of a manually entered URL? And then auto-magically "fix" it? Sometimes it is better to just try to use what the user entered and if it doesn't work, present an error message about what is acceptable for a URL.

Just another regex example... I'm sure that other monks can provide even better regex'es, but specifying the problem as clearly as you can is important.

my @urls = ('tp://www.foo.com/' , '://www.foo.com', 
            'http//:www.foo.com', 'www.foo.com',
            'xxx.tv', 'http//:xxx.tv', 'tp:xx.tv'); 

foreach (@urls)
{
   s/^(.*?)(\w+\.)/$2/;
   print "http://$_\n";
}

__END__
prints:
http://www.foo.com/
http://www.foo.com
http://www.foo.com
http://www.foo.com
http://xxx.tv
http://xxx.tv
http://xx.tv
[download]