diff --git a/src/preprocess.rs b/src/preprocess.rs
index 6ea4127..3a6a0cd 100644
--- a/src/preprocess.rs
+++ b/src/preprocess.rs
@@ -3,12 +3,14 @@ use once_cell::unsync::Lazy;
 use regex::Regex;
 use std::env;
 
-const ENV_SET: Lazy<Regex> = Lazy::new(|| Regex::new(r#"(?P<key>\w+)=(?P<value>\w*)"#).unwrap());
+const ENV_SET: Lazy<Regex> = Lazy::new(|| Regex::new(r#"^(?P<key>\w+)=(?P<value>\w*)$"#).unwrap());
 const ENV_VARIABLE: Lazy<Regex> = Lazy::new(|| Regex::new(r"(\\\$|\$(?P<env>\w+))").unwrap());
 
 /// Returns `Ok(Some(...))` if a command should be triggered after calling this functions. If it
 /// returns `Ok(None)`, do not execute a command.
 pub fn preprocess(mut line: String) -> Result<Option<String>, ShellError> {
+    // check if the input line has `key=value` syntax. if so, set an env variable with `key` as key
+    // and `value` as value
     if let Some(capture) = ENV_SET.captures(&line) {
         let key = capture.name("key").unwrap().as_str();
         let value = capture
@@ -19,6 +21,9 @@ pub fn preprocess(mut line: String) -> Result<Option<String>, ShellError> {
         return Ok(None);
     }
 
+    // resolve env variables. if the input line contains `$key` and a env variable with the name
+    // `key` exists too, resolve `$key` to the variable value. if `key` env variable does not exist,
+    // simply resolve it with an empty string
     for capture in ENV_VARIABLE.captures_iter(&line.clone()) {
         let variable_name = capture.name("env").unwrap().as_str();
         if variable_name.is_empty() {