Get all words between two markers in a string

I need to get the list of all value names in a given module.
So I put the contents of the mli in a string, and now I need
to do some regexp manipulation on it, selecting all
words between "val" and ":"  that's the part where
I'm stuck.

So what would be nice here is a grab_value_names:string->string
function, such that

grab_value_names "**** val get : ***** val set : **** val make : ***"

returns ["get";"set";"make"].


I'm not sure about how to do this with the Str module. I thought first
of cleaning everything between a ":" and "val" ; I tried the following
which doesn't work :

let cleanup s=Str.global_replace (Str.regexp ":[.]+val") ":\nval" s;;