RE: How to add prefix before running interactive regexpsearch(isearch-forward-regexp)?

[Drew Adams]

> Store that "prefix" in registers for easy insertion

Or copy the string to the kill-ring and then yank it into Isearch using `M-y'.

Or copy it as the secondary selection and yank it using `C-M-y'. (You need
library second-sel.el for this.) The secondary selection doesn't get overwritten
when you update the clipboard or the kill ring.

http://www.emacswiki.org/emacs/SecondarySelection