diff --git a/imap_filter/imap_filter.py b/imap_filter/imap_filter.py
index 78cfdf2..4382943 100644
--- a/imap_filter/imap_filter.py
+++ b/imap_filter/imap_filter.py
@@ -151,11 +151,13 @@ class ImapFilter:
                 condition = f'allof (header :contains "{field}" ["{'", "'.join(mfilter['words'])}"])'
             search = f"^.*  # imap_filter: rule name 'imap_filter_INBOX_{mfilter['name']}'"
             replacement = f'if {condition} {{ fileinto "{mfilter['destination']}"; stop;}}  # imap_filter: rule name \'imap_filter_INBOX_{mfilter['name']}\''
-            new_content = re.sub(search, replacement, content)
-            if content == new_content:
-                content += f"{replacement}\n"
+            match = re.search(search, content)
+            if match:
+                new_content = re.sub(search, replacement, content)
+                if content != new_content:
+                    content = f"{new_content}\n"
             else:
-                content = f"{new_content}\n"
+                content += f"{replacement}\n"
         with open(self.config['sieve_scripts_path'], 'w', encoding='UTF-8') as sieve_script:
             sieve_script.write(content)
         return True