diff --git a/crates/shim/Cargo.toml b/crates/shim/Cargo.toml
index 55e55f27..bcc1d1b7 100644
--- a/crates/shim/Cargo.toml
+++ b/crates/shim/Cargo.toml
@@ -36,7 +36,6 @@ signal-hook = "0.3.13"
 oci-spec = "0.6.0"
 prctl = "1.0.0"
 page_size = "0.6.0"
-regex = "1"
 
 containerd-shim-protos = { path = "../shim-protos", version = "0.5.0" }
 
diff --git a/crates/shim/src/mount.rs b/crates/shim/src/mount.rs
index f823a50a..9c93c8c6 100644
--- a/crates/shim/src/mount.rs
+++ b/crates/shim/src/mount.rs
@@ -29,7 +29,6 @@ use log::error;
 use nix::mount::{mount, MsFlags};
 #[cfg(target_os = "linux")]
 use nix::unistd::{fork, ForkResult};
-use regex::Regex;
 
 use crate::error::{Error, Result};
 #[cfg(not(feature = "async"))]
@@ -239,49 +238,40 @@ fn options_size(options: &[String]) -> usize {
     options.iter().fold(0, |sum, x| sum + x.len())
 }
 
-fn longest_common_prefix(dirs: &[String]) -> Option<String> {
+fn longest_common_prefix(dirs: &[String]) -> &str {
     if dirs.is_empty() {
-        return None;
-    }
-    if dirs.len() == 1 {
-        if dirs[0].is_empty() {
-            return None;
-        }
-        return Some(dirs[0].to_string());
+        return "";
     }
 
-    let min = dirs.iter().min().unwrap();
-    let max = dirs.iter().max().unwrap();
-    let min_chars = min.chars().collect::<Vec<char>>();
-    let max_chars = max.chars().collect::<Vec<char>>();
-    let mut i = 0;
-    while i < min_chars.len() && i < max_chars.len() {
-        if min_chars[i] != max_chars[i] {
-            if i == 0 {
-                return None;
+    let first_dir = &dirs[0];
+
+    for (i, byte) in first_dir.as_bytes().iter().enumerate() {
+        for dir in dirs {
+            if dir.as_bytes().get(i) != Some(byte) {
+                let mut end = i;
+                // guaranteed not to underflow since is_char_boundary(0) is always true
+                while !first_dir.is_char_boundary(end) {
+                    end -= 1;
+                }
+
+                return &first_dir[0..end];
             }
-            return Some(min[0..i].to_string());
         }
-        i += 1;
     }
 
-    if min.is_empty() {
-        None
-    } else {
-        Some(min.to_string())
-    }
+    first_dir
 }
 
 // NOTE: the snapshot id is based on digits.
-// in order to avoid to get snapshots/x, shoule be back to parent dir.
+// in order to avoid to get snapshots/x, should be back to parent dir.
 // however, there is assumption that the common dir is ${root}/io.containerd.v1.overlayfs/snapshots.
 #[cfg(target_os = "linux")]
 fn trim_flawed_dir(s: &str) -> String {
-    let r = Regex::new(r"((/[^/]+)+/)([^/]*)").unwrap();
-    r.replace(s, "$1").to_string()
+    s[0..s.rfind('/').unwrap_or(0) + 1].to_owned()
 }
 
 #[cfg(target_os = "linux")]
+#[derive(Default)]
 struct LowerdirCompactor {
     options: Vec<String>,
     lowerdirs: Option<Vec<String>>,
@@ -293,8 +283,7 @@ impl LowerdirCompactor {
     fn new(options: &[String]) -> Self {
         Self {
             options: options.to_vec(),
-            lowerdirs: None,
-            lowerdir_prefix: None,
+            ..Self::default()
         }
     }
 
@@ -317,9 +306,7 @@ impl LowerdirCompactor {
             .as_ref()
             .filter(|x| x.len() > 1)
             .map(|x| longest_common_prefix(x))
-            .unwrap_or(None)
-            .filter(|x| x != "/")
-            .map(|x| trim_flawed_dir(&x))
+            .map(trim_flawed_dir)
             .filter(|x| !x.is_empty() && x != "/");
         self
     }
@@ -620,6 +607,8 @@ mod tests {
     #[test]
     fn test_trim_flawed_dir() {
         let mut tcases: Vec<(&str, String)> = Vec::new();
+        tcases.push(("/", "/".to_string()));
+        tcases.push(("/foo", "/".to_string()));
         tcases.push(("/.foo-_bar/foo", "/.foo-_bar/".to_string()));
         tcases.push(("/.foo-_bar/foo/", "/.foo-_bar/foo/".to_string()));
         tcases.push(("/.foo-_bar/foo/bar", "/.foo-_bar/foo/".to_string()));
@@ -632,21 +621,21 @@ mod tests {
 
     #[test]
     fn test_longest_common_prefix() {
-        let mut tcases: Vec<(Vec<String>, Option<String>)> = Vec::new();
-        tcases.push((vec![], None));
-        tcases.push((vec!["foo".to_string()], Some("foo".to_string())));
-        tcases.push((vec!["foo".to_string(), "bar".to_string()], None));
+        let mut tcases: Vec<(Vec<String>, String)> = Vec::new();
+        tcases.push((vec![], "".to_string()));
+        tcases.push((vec!["foo".to_string()], "foo".to_string()));
+        tcases.push((vec!["foo".to_string(), "bar".to_string()], "".to_string()));
         tcases.push((
             vec!["foo".to_string(), "foo".to_string()],
-            Some("foo".to_string()),
+            "foo".to_string(),
         ));
         tcases.push((
             vec!["foo".to_string(), "foobar".to_string()],
-            Some("foo".to_string()),
+            "foo".to_string(),
         ));
         tcases.push((
             vec!["foo".to_string(), "".to_string(), "foobar".to_string()],
-            None,
+            "".to_string(),
         ));
         for (case, expected) in tcases {
             let res = longest_common_prefix(&case);