fix(functions/switch_ubuntu_local_mirror): Fix missing validation of the region code

Signed-off-by: 林博仁(Buo-ren Lin) <[email protected]>

Author: brlin-tw

functions.sh

@@ -634,12 +634,21 @@ switch_ubuntu_local_mirror(){
             'Warning: Unable to detect the local region code, falling back to the current settings.\n' \
             1>&2
         return 0
-    else
+    fi
+
+    local regex_region_code='^[a-z]{2,3}$'
+    if ! [[ "${region_code}" =~ ${regex_region_code} ]]; then
         printf \
-            'Info: Local region code determined to be "%s".\n' \
-            "${region_code}"
+            'Error: Local region code "%s" is not valid.\n' \
+            "${region_code}" \
+            1>&2
+        return 2
     fi
 
+    printf \
+        'Info: Local region code determined to be "%s".\n' \
+        "${region_code}"
+
     if test -n "${region_code}"; then
         printf \
             'Info: Checking whether the local Ubuntu archive mirror exists...\n'