I do not have all the commands memorised. But how emacs completes changes upon where you hit tab. if you type M-x re-bu TAB, it will show revert-buffer, re-builder, etc. If you hit TAB when cursor is at the beginning, like M-x buffer M-b TAB, it will show any command with buffer in it. Then you can C-x o to the Completions buffer and use any search tool on it, e.g. isearch-forward-regexp.