diff --git a/src/settings/mod.rs b/src/settings/mod.rs
index 5814dd6..c29f780 100644
--- a/src/settings/mod.rs
+++ b/src/settings/mod.rs
@@ -284,24 +284,26 @@ impl Settings {
         RULESET_CURRENT_VERSION.to_string()
     }
 
+    // TODO make the return type a Result
     fn update_rules(&self) -> Vec<(Regex, String)> {
         let mut rules = self.rules.clone();
 
+        // TODO Optimize that shit
+        // TODO break categories in different rules
         for (category, content) in &self.categories {
             let mut temp_rules: Vec<(Regex, String)> = Vec::new();
             for (from, to) in &rules {
                 let from = Regex::new(
                     from.to_string()
                         .replace(
-                            category.as_str(),
+                            format!("%{}", category).as_str(),
                             format!("[{}]", content).as_str(),
                         )
-                        .replace("%", "")
                         .as_str(),
                 );
                 let to = to.to_string().replace(
-                    category,
-                    format!("[{}]", content).replace("%", "").as_str(),
+                    format!("%{}", category).as_str(),
+                    format!("[{}]", content).as_str(),
                 );
                 temp_rules.push((from, to));
             }
@@ -334,8 +336,7 @@ impl Settings {
     /// // set some input
     /// let _output = settings.apply(input);
     /// ```
-    pub fn apply(&self, s: String) -> String {
-        // Replace all `%C`s by their equivalent
+    pub fn apply(&self, s: String) -> std::result::Result<String, String> {
         let rules = self.update_rules();
         let mut s = s.clone();
         debug!("===============================================");
@@ -345,7 +346,7 @@ impl Settings {
             s = from.replace_all(&s, to.as_str()).to_string();
             debug!("new: {}", s);
         }
-        s
+        Ok(s)
     }
 }