Add match?

lib/drops/predicates.ex

@@ -88,6 +88,12 @@ defmodule Drops.Predicates do
   def size?(size, value) when is_list(value) and length(value) != size,
     do: {:error, {:size?, [size, value]}}
 
+  def match?(regexp, value) do
+    if String.match?(value, regexp),
+      do: {:ok, value},
+      else: {:error, {:match?, [regexp, value]}}
+  end
+
   def max_size?(size, value) when is_map(value) and map_size(value) <= size,
     do: {:ok, value}
 

test/contract/predicates_test.exs

@@ -472,4 +472,21 @@ defmodule Drops.PredicatesTest do
                contract.conform(%{test: [1, 2]})
     end
   end
+
+  describe "match?/2" do
+    contract do
+      schema do
+        %{required(:test) => type(:string, match?: ~r/\d+/)}
+      end
+    end
+
+    test "returns success when the value matches the regexp", %{contract: contract} do
+      assert {:ok, %{test: "312"}} = contract.conform(%{test: "312"})
+    end
+
+    test "returns success when the value doesn't match the regexp", %{contract: contract} do
+      assert {:error, [{:error, {:match?, [:test], [~r/\d+/, "Hello"]}}]} =
+               contract.conform(%{test: "Hello"})
+    end
+  end
 end