You can try a negative lookbehind assertion (in perlman:perlre), but it has to be a fixed length pattern, and so won't work if there's spaces around the '=' (so you can first fix that):

s/href\s*=\s*/href=/ig;
s,(?<!(?i:href=))((?:http|ftp|news)://\S*),<a href="$1">$1</a>,g;

You might try to think of something besides '\S*' also, in case
the user puts in a period after the href, or puts it between
parenthesis or ???.
[download]

Update: Actually, if the user DID already make an href, they may or may not have put quotes around the '=' in the url, so maybe the easiest thing to do is rewrite all urls:

# Now its starting to get ugly!
$str =~ s#((?i:<a\s+href\s*=\s*"?)?(?:http|ftp|news)://[^"]+)#substr($
+1,0,1) eq '<' ? $1 : qq!<a href="$1"
>$1</a>!#eg;
[download]

I think that's right now (and you still may want to replace that '[^"]' with something else) :-)

Actually, even that may not work if there's other attributes besides 'href' in the tag, but I'm done. The rest is up to you :-)