diff --git a/src/app_state.rs b/src/app_state.rs
index 5979150f..7de17543 100644
--- a/src/app_state.rs
+++ b/src/app_state.rs
@@ -20,7 +20,7 @@ use crate::{
     embedded::EMBEDDED_FILES,
     exercise::{Exercise, RunnableExercise},
     info_file::ExerciseInfo,
-    term::{self, CheckProgressVisualizer},
+    term::{self, CheckProgressVisualizer}, url_replacer::UrlReplacer,
 };
 
 const STATE_FILE_NAME: &str = ".rustlings-state.txt";
@@ -68,6 +68,7 @@ impl AppState {
     pub fn new(
         exercise_infos: Vec<ExerciseInfo>,
         final_message: String,
+        base_url: Option<String>,
     ) -> Result<(Self, StateFileStatus)> {
         let cmd_runner = CmdRunner::build()?;
         let mut state_file = OpenOptions::new()
@@ -80,6 +81,13 @@ impl AppState {
                 format!("Failed to open or create the state file {STATE_FILE_NAME}")
             })?;
 
+        // replacer for rustbook url
+        let url_replacer = if let Some(url) = &base_url {
+            Some(UrlReplacer::new(&url))
+        } else {
+            None
+        };
+
         let dir_canonical_path = term::canonicalize("exercises");
         let mut exercises = exercise_infos
             .into_iter()
@@ -90,7 +98,11 @@ impl AppState {
                 let path = exercise_info.path().leak();
                 let name = exercise_info.name.leak();
                 let dir = exercise_info.dir.map(|dir| &*dir.leak());
-                let hint = exercise_info.hint.leak().trim_ascii();
+                let mut hint = exercise_info.hint.leak().trim_ascii();
+
+                if let Some(replacer) = &url_replacer {
+                    hint = replacer.replace(&hint).leak();
+                }
 
                 let canonical_path = dir_canonical_path.as_deref().map(|dir_canonical_path| {
                     let mut canonical_path;
diff --git a/src/main.rs b/src/main.rs
index c27bfa89..2ee376b4 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -8,7 +8,7 @@ use std::{
 };
 use term::{clear_terminal, press_enter_prompt};
 
-use self::{app_state::AppState, dev::DevCommands, info_file::InfoFile, url_replacer::UrlReplacer};
+use self::{app_state::AppState, dev::DevCommands, info_file::InfoFile};
 
 mod app_state;
 mod cargo_toml;
@@ -99,6 +99,7 @@ fn main() -> Result<ExitCode> {
     let (mut app_state, state_file_status) = AppState::new(
         info_file.exercises,
         info_file.final_message.unwrap_or_default(),
+        args.base_url,
     )?;
 
     // Show the welcome message if the state file doesn't exist yet.
@@ -186,13 +187,7 @@ fn main() -> Result<ExitCode> {
                 app_state.set_current_exercise_by_name(&name)?;
             }
 
-            let hint = app_state.current_exercise().hint;
-            if let Some(base_url) = args.base_url {
-                let replacer = UrlReplacer::new(base_url);
-                println!("{}", replacer.replace(hint));
-            } else {
-                println!("{}", hint);
-            };
+            println!("{}", app_state.current_exercise().hint);
         }
         // Handled in an earlier match.
         Some(Subcommands::Init | Subcommands::Dev(_)) => (),
diff --git a/src/url_replacer.rs b/src/url_replacer.rs
index 97977305..b32a8495 100644
--- a/src/url_replacer.rs
+++ b/src/url_replacer.rs
@@ -5,18 +5,20 @@ pub struct UrlReplacer {
 const EN_BASE_URL: &str = "https://doc.rust-lang.org/book";
 
 impl UrlReplacer {
-    pub fn new(mut base_url: String) -> Self {
-        base_url = if base_url.ends_with('/') {
+    /// this fn will trim url end with '/'
+    pub fn new(base_url: &String) -> Self {
+        let url = if base_url.ends_with('/') {
             base_url.trim_end_matches('/').to_owned()
         } else {
-            base_url
+            base_url.clone()
         };
 
         Self {
-            base_url
+            base_url: url
         }
     }
 
+    /// replace rustbook url
     pub fn replace(&self, hint: &str) -> String {
         hint.replace(EN_BASE_URL, &self.base_url)
     }
@@ -30,7 +32,7 @@ mod test {
 
     #[test]
     fn non_rustbook_url() {
-        let replacer = UrlReplacer::new(String::from(TEST_DOMAIN));
+        let replacer = UrlReplacer::new(&String::from(TEST_DOMAIN));
 
         let hint = "\
 hints (...) lines (...)
@@ -42,7 +44,7 @@ link: https://example.com/ch03-02-data-types.html";
 
     #[test]
     fn replace_rustbook_url() {
-        let replacer = UrlReplacer::new(String::from(TEST_DOMAIN));
+        let replacer = UrlReplacer::new(&String::from(TEST_DOMAIN));
 
         let hint = "\
 hints (...) lines (...)
@@ -55,7 +57,10 @@ link: https://doc.rust-kr.org/ch03-02-data-types.html", replacer.replace(hint));
 
     #[test]
     fn trim_end_with_slash() {
-        let replacer = UrlReplacer::new(String::from(TEST_DOMAIN));
+        let mut domain = String::from(TEST_DOMAIN);
+        domain.push('/');
+
+        let replacer = UrlReplacer::new(&domain);
 
         assert_eq!(TEST_DOMAIN, replacer.base_url);
     }