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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 ???.

# 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;