diff --git a/data/structures.yml b/data/structures.yml
index ccc09e8..37d96bb 100644
--- a/data/structures.yml
+++ b/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
diff --git a/spec/ibandit/iban_spec.rb b/spec/ibandit/iban_spec.rb
index 91c6921..8e5e516 100755
--- a/spec/ibandit/iban_spec.rb
+++ b/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