add url validator

url.go

@@ -0,0 +1,17 @@
+package validate
+
+import "net/url"
+
+// Validation error.
+var (
+	ErrInvalidURL Error = NewError("invalid URL")
+)
+
+// URL validates a URL.
+func URL(value string) error {
+	if _, err := url.ParseRequestURI(value); err != nil {
+		return ErrInvalidURL
+	}
+
+	return nil
+}

url_test.go

@@ -0,0 +1,31 @@
+package validate
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestURL(t *testing.T) {
+	type TestCase struct {
+		Input string
+		Err   error
+	}
+
+	testCases := []TestCase{
+		{Input: "http://example.com"},
+		{Input: "http://subdomain.example.com"},
+		{Input: "http://www.example.com/some-page.html"},
+		{Input: "subdomain.com", Err: ErrInvalidURL},
+		{Input: "not a url", Err: ErrInvalidURL},
+	}
+
+	for n, tc := range testCases {
+		t.Logf("(%d) Testing %q", n, tc.Input)
+
+		err := URL(tc.Input)
+
+		if !errors.Is(err, tc.Err) {
+			t.Errorf("Expected error %v, got %v", tc.Err, err)
+		}
+	}
+}