diff --git a/tools/validate-indentation.sh b/tools/validate-indentation.sh
index de2f20c6f8..f651e152bd 100755
--- a/tools/validate-indentation.sh
+++ b/tools/validate-indentation.sh
@@ -3,8 +3,9 @@
 RES=0
 
 for dockerfile in "$@"; do
-    if grep -qE '^\s+[A-Z]+\s' "$dockerfile"; then
+    if grep -qE '^\s+[[:upper:]]+\s+' "$dockerfile"; then
         echo "ERROR: $dockerfile has indented Dockerfile instruction" >&2
+        grep -E '^\s+[[:upper:]]+\s+' "$dockerfile"
         RES=1
     fi
 done