Solve "make check-per" failed on Lubuntu

examples/sample.makefile.regen

@@ -113,7 +113,7 @@ set +x
 	echo '	done; done; fi'
 	echo "	@if test -f sample-${ASN1PDU}-1.per ; then \\"
 	echo "	for f in sample-*-[1-9].per; do \\"
-	echo '	pdu=`echo $$f | sed -E -e "s/sample-([A-Za-z-]+)-[0-9].*/\1/"`; \'
+	echo '	pdu=`echo $$f | sed -E -e "s/sample-([A-Za-z-]+)-[0-9].*/\\1/"`; \'
 	echo '	for b in 1 17 33 980 8192; do \'
 	echo '	echo "Recoding $$f into DER into XER and back ($$b)..."; \'
 	echo '	./${TARGET} -b $$b -p $$pdu -iper -oder $$f > ./.tmp.1.$$$$ || exit 3; \'