diff --git a/src/stringcalculator.py b/src/stringcalculator.py
index 4b8e76f33e53fb4ef8cdc900d5930821fb39baed..fc7e89a3c15a18957c55241afea7767fcad48f34 100644
--- a/src/stringcalculator.py
+++ b/src/stringcalculator.py
@@ -11,7 +11,11 @@ class StringCalculator(IStringCalculator):
             if "\n" not in numbers:
                 raise ValueError("Ungültiges Format: Nicht vollständig")
             delimiter_end_index = numbers.index("\n")
-            delimiter = numbers[2:delimiter_end_index]
+            delimiter_part = numbers[2:delimiter_end_index]
+            if delimiter_part.startswith("[") and delimiter_part.endswith("]"):
+                delimiter = delimiter_part[1:-1]
+            else:
+                delimiter = delimiter_part
             numbers = numbers[delimiter_end_index + 1:]
             numbers = numbers.replace(delimiter, ",")
 
diff --git a/tests/test_stringclaculator.py b/tests/test_stringclaculator.py
index 002343c092e7e4d78fcb594ea61197483a496f16..e2e765bd59b8d7d50121700aa53d98082b57a32f 100644
--- a/tests/test_stringclaculator.py
+++ b/tests/test_stringclaculator.py
@@ -74,6 +74,10 @@ class TestStringCalculator(unittest.TestCase):
         self.assertEqual(self.calculator.add("2,1001"), 2)
         self.assertEqual(self.calculator.add("1002,50200"), 0)
 
+    def test_add_with_custom_delimiter_multiple_characters(self):
+        self.assertEqual(self.calculator.add("//[**]\n1**2**3"), 6)
+        self.assertEqual(self.calculator.add("//[###]\n10###20###30"), 60)
+
 if __name__ == "__main__":
     unittest.main()