I don't think that the caret ^ is going to work if you actually have the http:// as part of the field. I suspect that you could get exactly what you want by using conditional look ahead and/or look behind in your regex. But those things make my head hurt - I'd rather write a custom Splunk command! (And that's not trivial.)
You might take the regex problem to a forum that specializes in regexes or maybe Perl.
However, this would also match something like /shopping/welcome.html, /politics/welcome.html.
Can the regex be modified even further so that it looks for a format which has at least 1 or more character from each Charset, [a-z][A-Z][0-9]. I need to match it in such a way that it has at least 1 or more character from each Character Classes.