builder/amazon: numbers are valid in ami name

builder/amazon/common/ami_config.go

@@ -73,9 +73,12 @@ func (c *AMIConfig) Prepare(ctx *interpolate.Context) []error {
 		errs = append(errs, fmt.Errorf("AMIName must be between 3 and 128 characters long"))
 	}
 
-	amiNameRe := regexp.MustCompile(`^[a-zA-Z().\-/_]+$`)
+	amiNameRe := regexp.MustCompile(`^[0-9a-zA-Z().\-/_]+$`)
 	if !amiNameRe.MatchString(c.AMIName) {
-		errs = append(errs, fmt.Errorf("AMIName should only contain letters, numbers, '(', ')', '.', '-', '/' and '_'"))
+		errs = append(errs, fmt.Errorf("AMIName should only contain letters,"+
+			" numbers, '(', ')', '.', '-', '/' and '_'. You can use the "+
+			"`clean_ami_name` template filter to automatically clean your ami "+
+			"name."))
 	}
 
 	if len(errs) > 0 {

builder/amazon/common/ami_config_test.go

@@ -102,4 +102,9 @@ func TestAMINameValidation(t *testing.T) {
 		t.Fatal("expected 'foobar' to be a valid ami name")
 	}
 
+	c.AMIName = `xyz-base-2017-04-05-1934`
+	if err := c.Prepare(nil); err != nil {
+		t.Fatalf("expected `xyz-base-2017-04-05-1934` to pass validation.")
+	}
+
 }