Emacs regexp count occurrences

Question

I'm looking for the fastest routine (not interactively) to get the number of matches of a regexp in a string.

Something like

(count-occurrences "a" "alabama")
=> 4

Answer 1

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

Answer 2

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))