Regexp Search

1. [C-M-s] (or M-x isearch-forward-regexp) : incremental forward search matching regexp
2. [C-M-r] (or M-x isearch-backward-regexp) : incremental backward search matching regexp

alternative

1. invoke incremental search by either C-s (isearch-forward) or C-r (isearch-backward)
2. note the I-search: prompt in the echo area
3. type M-r
4. note the Pending regexp I-search: prompt in the echo area