diff --git a/ROMFS/px4fmu_common/init.d-posix/rcS b/ROMFS/px4fmu_common/init.d-posix/rcS
index 804b059fcb..91c9cb7c8f 100644
--- a/ROMFS/px4fmu_common/init.d-posix/rcS
+++ b/ROMFS/px4fmu_common/init.d-posix/rcS
@@ -40,8 +40,8 @@ if [ "$PX4_SIM_MODEL" == "shell" ]; then
 	set RUN_MINIMAL_SHELL yes
 else
 	# Find the matching Autostart ID (file name has the form: [0-9]+_${PX4_SIM_MODEL})
-	# shellcheck disable=SC2010 #(the file names don't contain spaces)
-	REQUESTED_AUTOSTART=$(ls "$SCRIPT_DIR" | grep -oP '^[0-9]+(?=_'${PX4_SIM_MODEL}'$)')
+	# shellcheck disable=SC2012 #(the file names don't contain spaces)
+	REQUESTED_AUTOSTART=$(ls "$SCRIPT_DIR" | sed -n 's/^\([0-9][0-9]*\)_'${PX4_SIM_MODEL}'$/\1/p')
 	if [ -z "$REQUESTED_AUTOSTART" ]; then
 		echo "Error: Unknown model '$PX4_SIM_MODEL'"
 		exit -1