Add some string matching utils (#263)

Splitting this out from #262 for ease of review; please suggest more
maintainable / idomatic ways of implementing these functions if there
are such

src/helpers.ml

@@ -2,12 +2,24 @@ open Base
 
 let f = Printf.sprintf
 
-let string_match ~regexp string =
+let string_match ~regexp ?(pos = 0) string =
   try
-    let _ = Str.search_forward (Str.regexp regexp) string 0 in
+    let (_ : int) = Str.search_forward (Str.regexp regexp) string pos in
     true
   with Stdlib.Not_found -> false
 
+let rec fold_string_matches ~regexp ~f ~init ?(pos = 0) string =
+  if string_match ~regexp ~pos string then
+    let pos = Str.match_end () in
+    f (fun () -> fold_string_matches ~regexp ~f ~init ~pos string)
+  else init
+
+let map_string_matches ~regexp ~f string =
+  fold_string_matches ~regexp ~f:(fun rest -> let v = f () in v :: rest ()) ~init:[] string
+
+let iter_string_matches ~regexp ~f string =
+  fold_string_matches ~regexp ~f:(fun rest -> f () ; rest ()) ~init:() string
+
 let pr_from_branch branch =
   if string_match ~regexp:"^pr-\\([0-9]*\\)$" branch then
     (Some (Str.matched_group 1 branch |> Int.of_string), "pull request")

src/helpers.mli

@@ -1,6 +1,13 @@
 val f : ('a, unit, string) format -> 'a
 
-val string_match : regexp:string -> string -> bool
+val string_match : regexp:string -> ?pos:int -> string -> bool
+
+val fold_string_matches :
+  regexp:string -> f:((unit -> 'a) -> 'a) -> init:'a -> ?pos:int -> string -> 'a
+
+val map_string_matches : regexp:string -> f:(unit -> 'a) -> string -> 'a list
+
+val iter_string_matches : regexp:string -> f:(unit -> unit) -> string -> unit
 
 val pr_from_branch : string -> int option * string