test: alternation parsing in various syntaxes

lib_test/expect/import.ml

@@ -80,3 +80,5 @@ let rec sexp_of_dyn (t : Re_private.Dyn.t) : Base.Sexp.t =
          | List [] -> None
          | sexp -> Some (Base.Sexp.List [ Atom name; sexp ])))
 ;;
+
+let print_dyn dyn = sexp_of_dyn dyn |> Base.Sexp.to_string_hum |> print_endline

lib_test/expect/test_alternation.ml

@@ -0,0 +1,48 @@
+open Import
+module Ast = Re_private.Ast
+
+let test f string =
+  match f string with
+  | Ok res -> print_dyn (Ast.to_dyn res)
+  | Error _ -> assert false
+;;
+
+let%expect_test "pcre" =
+  test Re.Pcre.re_result "(a|b|c)";
+  [%expect
+    {|
+    (Group
+     (Set
+      (Cast (Alternative (Cast (Alternative (Cset 97) (Cset 98))) (Cset 99)))))
+    |}]
+;;
+
+let%expect_test "emacs" =
+  test Re.Emacs.re_result {|\(a\|b\|c\)|};
+  [%expect
+    {|
+    (Group
+     (Set
+      (Cast (Alternative (Cast (Alternative (Cset 97) (Cset 98))) (Cset 99)))))
+    |}]
+;;
+
+let%expect_test "perl" =
+  test Re.Perl.re_result "(a|b|c)";
+  [%expect
+    {|
+    (Group
+     (Set
+      (Cast (Alternative (Cast (Alternative (Cset 97) (Cset 98))) (Cset 99)))))
+    |}]
+;;
+
+let%expect_test "posix" =
+  test Re.Posix.re_result "(a|b|c)";
+  [%expect
+    {|
+    (Group
+     (Set
+      (Cast (Alternative (Cast (Alternative (Cset 97) (Cset 98))) (Cset 99)))))
+    |}]
+;;