diff --git a/output.txt b/output.txt
new file mode 100644
index 0000000..d40e5a2
--- /dev/null
+++ b/output.txt
@@ -0,0 +1,2 @@
+Are you nobody, too?
+How dreary to be somebody!
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..f23dccc
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,114 @@
+use std::error::Error;
+use std::fs;
+use std::env;
+
+pub struct Config {
+    pub query: String,
+    pub filename: String,
+    pub ignore_case: bool,
+}
+
+impl Config {
+    pub fn build(
+        mut args: impl Iterator<Item = String>,
+    ) -> Result<Config, &'static str> {
+        args.next();
+
+        let query = match args.next() {
+            Some(arg) => arg,
+            None => return Err("Didn't get a query string"),
+        };
+
+        let filename = match args.next() {
+            Some(arg) => arg,
+            None => return Err("Didn't get a file path"),
+        };
+
+        let ignore_case = env::var("IGNORE_CASE").is_ok();
+
+        Ok(Config {
+            query,
+            filename,
+            ignore_case,
+        })
+    }
+
+    pub fn new(args: &[String]) -> Result<Config, &'static str> {
+        if args.len() < 3 {
+            return Err("not enough arguments");
+        }
+        let query = args[1].clone();
+        let filename = args[2].clone();
+
+        let ignore_case = env::var("IGNORE_CASE").is_ok();
+
+        Ok(Config {
+            query,
+            filename,
+            ignore_case,
+        })
+    }
+}
+
+pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
+    let contents = fs::read_to_string(config.filename)?;
+
+    let results = if config.ignore_case {
+        search_case_insensitive(&config.query, &contents)
+    } else {
+        search(&config.query, &contents)
+    };
+
+    for line in results {
+        println!("{line}");
+    }
+
+    Ok(())
+}
+
+pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
+    contents
+        .lines()
+        .filter(|line| line.contains(query))
+        .collect()
+}
+
+pub fn search_case_insensitive<'a>(
+    query: &str,
+    contents: &'a str,
+) -> Vec<&'a str> {
+    contents
+        .lines()
+        .filter(|line| line.contains(&query.to_lowercase()))
+        .collect()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn case_sensitive() {
+        let query = "duct";
+        let contents = "\
+Rust:
+safe, fast, productive.
+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 c34444c..3a44bee 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,33 +1,17 @@
-use std::{env, fs};
+use std::{env, process};
 
+use minigrep::Config;
 
 fn main() {
     let args: Vec<String> = env::args().collect();
 
-    let config = Config::new(&args);
-    
-    println!("Searching for {}", config.query);
-    println!("In file {}", config.filename);
+    let config = Config::build(env::args()).unwrap_or_else(|err| {
+        eprintln!("Problem parsing arguments: {err}");
+        process::exit(1);
+    });
 
-    let contents = fs::read_to_string(config.filename)
-        .expect("Something went wrong reading the file");
-    
-    println!("With text:\n{}", contents);
-}
-
-struct Config {
-    query: String,
-    filename: String,
-}
-
-impl Config {
-    fn new(args: &[String]) -> Config {
-        if args.len() < 3 {
-            panic!("not anough arguments");
-        }
-        let query = args[1].clone();
-        let filename = args[2].clone();
-
-        Config {query, filename}
+    if let Err(e) = minigrep::run(config) {
+        eprintln!("Application error: {e}");
+        process::exit(1);
     }
 }