Emacs regexp count occurrences

how-many (aliased count-matches) does this, but works on buffers.

Here is one that works on strings:

(defun how-many-str (regexp str)
  (loop with start = 0
        for count from 0
        while (string-match regexp str start)
        do (setq start (match-end 0))
        finally return count))

Here is a more functional answer using recursion and an accumulator. As an added benefit, it does not use cl:

(defun count-occurences (regex string)
  (recursive-count regex string 0))

(defun recursive-count (regex string start)
  (if (string-match regex string start)
      (+ 1 (recursive-count regex string (match-end 0)))
    0))

count-matches does it interactively. Maybe a good place to start looking.