Allow digits only in US ACH account numbers

the validation rule allows an underscore
because that's the padding character for
account numbers shorter than 17 digits,
but that's OK to allow because we
remove leading padding characters.

data/structures.yml

@@ -902,8 +902,7 @@ US:
   :branch_code_length: 0
   :account_number_length: 17
   :bank_code_format: "\\d{9}"
-  # TODO: update account number format based on inputs from user research
-  :account_number_format: ".{1,17}"
+  :account_number_format: "\\A_*\\d{1,17}\\z"
   :national_id_length: 9
   :pseudo_iban_bank_code_length: 9
   :pseudo_iban_branch_code_length: 0

spec/ibandit/iban_spec.rb

@@ -724,7 +724,7 @@
       its(:to_s) { is_expected.to eq("") }
     end
 
-    context "when the input is an invalid US pseudo-IBAN" do
+    context "when the input pseudo-IBAN has an invalid US bank_code" do
       let(:arg) { "USZZ__012345601234567890123456" }
 
       it "is invalid and has the correct errors" do
@@ -733,6 +733,15 @@
           to eq(bank_code: "is the wrong length (should be 9 characters)")
       end
     end
+
+    context "when the input pseudo-IBAN has an invalid US account_number" do
+      let(:arg) { "USZZ965498456ABC01234567890123" }
+
+      it "is invalid and has an error populated" do
+        expect(subject.valid?).to eq(false)
+        expect(subject.errors).to eq(account_number: "is invalid")
+      end
+    end
   end
 
   describe "#to_s" do