diff --git a/scripts/check-repo.sh b/scripts/check-repo.sh
index b0f9846bd..69b428c2b 100755
--- a/scripts/check-repo.sh
+++ b/scripts/check-repo.sh
@@ -65,7 +65,7 @@ REPO_NAME="lab0-c"
 repo_html=$(curl -s "https://github.com/${REPO_OWNER}/${REPO_NAME}")
 
 # Extract the default branch name from data-default-branch="..."
-DEFAULT_BRANCH=$(echo "$repo_html" | grep -oP "/${REPO_OWNER}/${REPO_NAME}/blob/\K[^/]+(?=/LICENSE)" | head -n 1)
+DEFAULT_BRANCH=$(echo "$repo_html" | sed -nE "s#.*${REPO_OWNER}/${REPO_NAME}/blob/([^/]+)/LICENSE.*#\1#p" | head -n 1)
 
 if [ "$DEFAULT_BRANCH" != "master" ]; then
   echo "$DEFAULT_BRANCH"
@@ -80,8 +80,7 @@ curl -sSL -o "$temp_file" "$COMMITS_URL"
 
 # general grep pattern that finds commit links
 upstream_hash=$(
-  grep -Po 'href="[^"]*/commit/\K[0-9a-f]{40}' "$temp_file" \
-  | head -n 1
+  sed -nE 's/.*href="[^"]*\/commit\/([0-9a-f]{40}).*/\1/p' "$temp_file" | head -n 1
 )
 
 rm -f "$temp_file"
diff --git a/scripts/common.sh b/scripts/common.sh
index bb6353ebf..a41097b95 100644
--- a/scripts/common.sh
+++ b/scripts/common.sh
@@ -89,14 +89,14 @@ make_random_string() {
 
   if [ -z "$sub_str" ]; then
     # Produce an exact random string of length total_len
-    cat /dev/urandom | tr -dc 'a-z0-9' | head -c "$total_len"
+    cat /dev/urandom | LC_ALL=C tr -dc 'a-z0-9' | head -c "$total_len"
   else
     # Insert the substring at a random position
     local sub_len=${#sub_str}
     local rand_len=$(( total_len - sub_len ))
 
     local raw_rand
-    raw_rand=$(cat /dev/urandom | tr -dc 'a-z0-9' | head -c "$rand_len")
+    raw_rand=$(cat /dev/urandom | LC_ALL=C tr -dc 'a-z0-9' | head -c "$rand_len")
 
     local pos=$(( RANDOM % (rand_len + 1) ))
     echo "${raw_rand:0:pos}${sub_str}${raw_rand:pos}"