diff --git a/src/grub2/grub/grub.cfg b/src/grub2/grub/grub.cfg
index 2c74650..bc39ede 100644
--- a/src/grub2/grub/grub.cfg
+++ b/src/grub2/grub/grub.cfg
@@ -2534,12 +2534,13 @@ function hiperiso_boot {
     fi
 
     # Derive log_dir from ISO basename. GRUB script has no ${var##*/}
-    # bash pattern; replicate it with regexp: match ".*/" greedily then
-    # the final segment. If iso_path has no '/', hiso_base stays as the
-    # whole path (acceptable fallback).
+    # bash pattern; replicate it with regexp. Use --set N:VAR syntax
+    # (the same form the Ventoy modsrc uses elsewhere in this file) so
+    # the captured group is bound to a named variable in the same scope
+    # rather than to the positional $1 in the if-statement branch.
     set hiso_base="$iso_path"
-    if regexp '.*/([^/]+)' "$iso_path" ; then
-        set hiso_base="$1"
+    if regexp --set 1:hiso_tail '.*/([^/]+)$' "$iso_path" ; then
+        set hiso_base="$hiso_tail"
     fi
     set hiso_log_dir="/hiperiso/logs/${hiso_base}/"