diff --git a/src/lib.rs b/src/lib.rs
index 5e503e9..b004662 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,9 +1,11 @@
+use std::env;
 use std::error::Error;
 use std::fs;
 
 pub struct Config {
     pub query: String,
     pub file_path: String,
+    pub ignore_case: bool,
 }
 
 impl Config {
@@ -15,14 +17,26 @@ impl Config {
         let query = args[1].clone();
         let file_path = args[2].clone();
 
-        Ok(Config { query, file_path })
+        let ignore_case = env::var("IGNORE_CASE").is_ok();
+
+        Ok(Config {
+            query,
+            file_path,
+            ignore_case,
+        })
     }
 }
 
 pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
     let contents = fs::read_to_string(config.file_path)?;
 
-    for line in search(&config.query, &contents) {
+    let results = if config.ignore_case {
+        search_case_insensitive(&config.query, &contents)
+    } else {
+        search(&config.query, &contents)
+    };
+
+    for line in results {
         println!("{line}");
     }
 
@@ -41,18 +55,47 @@ pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
     results
 }
 
+pub fn search_case_insensitive<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
+    let query = query.to_lowercase();
+    let mut results = Vec::new();
+
+    for line in contents.lines() {
+        if line.to_lowercase().contains(&query) {
+            results.push(line)
+        }
+    }
+
+    results
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
     #[test]
-    fn one_result() {
+    fn case_sensitive() {
         let query = "duct";
         let contents = "\
 Rust:
 safe, fast, productive.
-Pick tthree.";
+Pick three.
+Duct tape.";
 
         assert_eq!(vec!["safe, fast, productive."], search(query, contents))
     }
+
+    #[test]
+    fn case_insensitive() {
+        let query = "rUsT";
+        let contents = "\
+Rust:
+safe, fast, productive.
+Pick three.
+Trust me.";
+
+        assert_eq!(
+            vec!["Rust:", "Trust me."],
+            search_case_insensitive(query, contents)
+        );
+    }
 }
diff --git a/src/main.rs b/src/main.rs
index aef3e0d..ddef18c 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -7,7 +7,7 @@ fn main() {
     let args: Vec<String> = env::args().collect();
 
     let config = Config::build(&args).unwrap_or_else(|err| {
-        println!("Problem parsing arguments: {err}");
+        eprintln!("Problem parsing arguments: {err}");
         process::exit(1);
     });
 
@@ -15,7 +15,7 @@ fn main() {
     // println!("In file {}", config.file_path);
 
     if let Err(e) = minigrep::run(config) {
-        println!("Application error: {e}");
+        eprintln!("Application error: {e}");
         process::exit(1);
     }
 }
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