Add is_valid_vin helper function

cli_demo.py

@@ -12,7 +12,20 @@ Administration) VIN decoding API via the helper module `nhtsa_api_call.py`.
 Usage:
     python cli_demo.py
 """
-import nhtsa_api_call
+import nhtsa_api_call, re
+
+def is_valid_vin (vin):
+    """
+    Check whether vin is valid.
+    A valid VIN is a VIN of 17 alphanumeric characters long and can not 
+    include I, O, Q. 
+    """
+    vin = vin.upper()
+    if len(vin) != 17:
+        return False 
+    if not re.match(r'^[A-HJ-NPR-Z0-9]{17}$', vin):
+        return False
+    return True
 
 def main():
     """
@@ -26,15 +39,15 @@ def main():
    
     while True:
         vin = input("Please enter a valid VIN or 0 (zero) to exit: \n").strip()
-        # empty vin condition
-        if not vin: 
-            print("⚠️  VIN cannot be empty. Please try again.\n")
-            continue
-        # exit condition
-        elif vin == "0":
-            print("👋 Exiting VIN lookup. Goodbye!")
+        # exit condition gets checked first
+        if vin == "0":
+            print("👋 You chose to exit VIN lookup. Goodbye!")
             break
-         # perform VIN lookup
+        # check if vin is valid, and if not print error message
+        if not is_valid_vin(vin): 
+            print("⚠️ Invalid VIN format. Must be 17 letters/numbers (no I, O, Q). Try again.\n")
+            continue
+         # vin is valid, perform VIN lookup
         else:
             year, make, model = nhtsa_api_call.get_vehicle_info(vin)
             if year and make and model: