#!/usr/bin/perl -p
s-^(((((((h?t)?)t)?p)?:)?/)?/)?(?=\w+\.)-http://-;
__END__
http://www.perlmonks.org/
ttp://www.perlmonks.org/
tp://www.perlmonks.org/
p://www.perlmonks.org/
://www.perlmonks.org/
//www.perlmonks.org/
/www.perlmonks.org/
www.perlmonks.org/
[download]

produces

http://www.perlmonks.org/
http://www.perlmonks.org/
http://www.perlmonks.org/
http://www.perlmonks.org/
http://www.perlmonks.org/
http://www.perlmonks.org/
http://www.perlmonks.org/
http://www.perlmonks.org/
[download]

(update:) Replace \. with [./] if you want to support intranet URLs like http://cvs/. Supporting alternate-port intranet URLs like http://wiki:8080/ with just [./:] would cause ftp://... to become http://ftp://... but you could consider (?=\w+([./]|:\d)).

#!/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]

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]

Your regex solution s/^(.*?)(\w+\.)/$2/ should work perfectly for this.

The mad scientist in me, though, is still wondering if there's a way to do this sort of thing abstractly: to provide a prefix for a string where the prefix may be only partially present. I'll think about it later. It's Friday.

---

Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
Nos autem praedicamus Christum crucifixum (1 Cor. 1:23) - The Cross Reference (My Blog)

I posted this in the wrong place first, so it may show twice

try something like this:

sub fix_URL {
    use URI;
    my $in = shift;
    my $url = URI->new($in);
    $url->scheme('http');
    print "input is: $in\n";
    print "fixed url is: $url\n";
}
[download]

perl> fix_URL 'tp://www.cnn.com'
input is: tp://www.cnn.com
fixed url is: http://www.cnn.com
perl>
[download]

excellent and elegant solution. the problem spec is still a bit hazy though. there's a whole host of urls that are not plain http://, do these also need pseudo-correction? e.g. https:, mailto:, javascript:, etc. even including browser specific ones such as those used in mozilla based browsers. if there's a need to fix all these other kinds of urls automatically, it would be pretty much impossible.
the hardest line to type correctly is: stty erase ^H

   I have a potentially malformed URL. It MAY be missing ... then again, it might be fine.

Honestly, I haven't the faintest clue what you're trying to do or why you think there is a problem. Here's what I'm able to gather from your post:

1. You have some source providing your script/program with URLs.
2. Your script/program is acting on that information.
3. Since your code is not always doing what is expected, you believe that there's a chance you're getting invalid URLs.

Sounds to me like it's time for debugging, which for me means to start adding print statements to figure out what's happening where. For example, print out the URLs, then look to see what kind of issues there are, and then develop a plan to deal with them. Doing this might help you figure out if you're getting invalid URLs and if so, how are they invalid, which in turn helps with figuring out the regex.

The only other idea right now is to find a module that does the URL validation for you.

"I haven't the faintest clue what you're trying to do"

I am trying to correct a potentially malformed "http://" at the beginning of a URL.

I am not in control of the URLs I am receiving. It is not for me to debug, it is simply for me to correct.

---

Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
Nos autem praedicamus Christum crucifixum (1 Cor. 1:23) - The Cross Reference (My Blog)

Haven't you ever copy-pasted urls with the mouse from a window to the browser title bar and find that you failed to select the h at the beginning?

use index to find "//", or failing that, "/". (It would be easier if all URLs started with www, but good luck on that). Depending on the results of the previous tests, prepend the appropriate substr from "http://".