Add utility function isValidSwissIbanNumber (#79)

Add utility function isValidSwissIbanNumber for issue #77

---------

Co-authored-by: Sandro C. <sc@neolution.ch>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- `isValidSwissIbanNumber` string utility function
 - `isValidSwissSocialSecurityNumber` string utility function
 
 ## [2.0.0] - 2025-07-29

src/lib/string.spec.ts

@@ -1,4 +1,12 @@
-import { isNullOrEmpty, isNullOrWhitespace, capitalize, uncapitalize, truncate, isValidSwissSocialSecurityNumber } from "./string";
+import {
+  isNullOrEmpty,
+  isNullOrWhitespace,
+  capitalize,
+  uncapitalize,
+  truncate,
+  isValidSwissIbanNumber,
+  isValidSwissSocialSecurityNumber,
+} from "./string";
 
 describe("string tests", () => {
   test.each([
@@ -121,6 +129,20 @@ describe("string tests", () => {
     expect(truncate(value, maxLength)).toBe(expected);
   });
 
+  test.each([
+    [null as unknown as string, false],
+    [undefined as unknown as string, false],
+    ["CH9300762011623852957", true],
+    ["CH93 0076 2011 6238 5295 7", true],
+    ["CH930076 20116238 5295 7", false],
+    ["CH93-0076-2011-6238-5295-7", false],
+    ["CH93 0000 0000 0000 0000 1", false],
+    ["ch93 0076 2011 6238 5295 7", false],
+    ["DE93 0076 2011 6238 5295 7", false],
+  ])("check if this swiss IBAN is valid or not", (unformattedIbanNumber, expected) => {
+    expect(isValidSwissIbanNumber(unformattedIbanNumber)).toBe(expected);
+  });
+
   test.each([
     [null as unknown as string, false],
     [undefined as unknown as string, false],

src/lib/string.ts

@@ -65,6 +65,51 @@ export function truncate(value: string | undefined, maxLength: number, suffix =
   return `${value.slice(0, maxLength)}${suffix}`;
 }
 
+/**
+ * Checks if the provided string is a valid swiss IBAN number
+ * @param ibanNumber The provided IBAN number to check
+ * Must be in one of the following formats:
+ * - "CHXX XXXX XXXX XXXX XXXX X" with whitespaces
+ * - "CHXXXXXXXXXXXXXXXXXXX" without whitespaces
+ * @returns The result of the IBAN number check
+ */
+export function isValidSwissIbanNumber(ibanNumber: string): boolean {
+  // 1. Reject null, undefined or whitespace-only strings
+  if (isNullOrWhitespace(ibanNumber)) {
+    return false;
+  }
+
+  // 2. Define allowed strict formats
+  //    - with spaces: "CHXX XXXX XXXX XXXX XXXX X"
+  const compactIbanNumberWithWhiteSpaces = new RegExp(/^CH\d{2}(?: \d{4}){4} \d{1}$/);
+  //    - without spaces: "CHXXXXXXXXXXXXXXXXXXX"
+  const compactIbanNumberWithoutWhiteSpaces = new RegExp(/^CH\d{19}$/);
+
+  // 3. Check if input matches one of the allowed formats
+  if (!compactIbanNumberWithWhiteSpaces.test(ibanNumber) && !compactIbanNumberWithoutWhiteSpaces.test(ibanNumber)) {
+    return false;
+  }
+
+  // 4. Remove all spaces to get a compact IBAN string
+  const compactIbanNumber = ibanNumber.replaceAll(" ", "");
+
+  // 5. Rearrange IBAN for checksum calculation
+  //    - move first 4 characters (CH + 2 check digits) to the end
+  const rearrangedIban = compactIbanNumber.slice(4) + compactIbanNumber.slice(0, 4);
+
+  // 6. Replace letters with numbers (A=10, B=11, ..., Z=35)
+  const numericStr = rearrangedIban.replaceAll(/[A-Z]/g, (ch) => (ch.codePointAt(0)! - 55).toString());
+
+  // 7. Perform modulo 97 calculation to validate IBAN
+  let restOfCalculation = 0;
+  for (const digit of numericStr) {
+    restOfCalculation = (restOfCalculation * 10 + Number(digit)) % 97;
+  }
+
+  // 8. IBAN is valid only if the remainder equals 1
+  return restOfCalculation === 1;
+}
+
 /**
  * Validation of social insurance number with checking the checksum
  * Validation according to https://www.sozialversicherungsnummer.ch/aufbau-neu.htm