That is best solved by changing the regex from nn(.*) to ^nn(.*) so that it only matches if the nn is at the START of the string.
Thank you. I was going go respond that in my case I don't actually want the anchor as often I am remembering a fragment of the name of the URL that could be anywhere in the string.
E.g. to find "Learning Elisp for better Emacs" I might simply type "elisp".
However, having tried it, it works, which seems to run counter to what I know about regexes...
EDIT: Wrong again, I am actually using "nn(^.*)" rather than "^nn(.*)" but this "nn(^.*)" still seems to get me the behaviour I want?