diff --git a/rules.mk b/rules.mk
index 5fd12a0..a6bae62 100644
--- a/rules.mk
+++ b/rules.mk
@@ -29,11 +29,11 @@ $(obj).depend:	$(src)Makefile $(TOPDIR)/config.mk $(SRCS) $(HOSTSRCS)
 		@rm -f $@
 		@touch $@
 		@for f in $(SRCS); do \
-			g=`basename $$f | sed -e 's/\(.*\)\.\w/\1.o/'`; \
+			g=`basename $$f | sed -e 's/\(.*\)\.[[:alnum:]_]/\1.o/'`; \
 			$(CC) -M $(CPPFLAGS) -MQ $(obj)$$g $$f >> $@ ; \
 		done
 		@for f in $(HOSTSRCS); do \
-			g=`basename $$f | sed -e 's/\(.*\)\.\w/\1.o/'`; \
+			g=`basename $$f | sed -e 's/\(.*\)\.[[:alnum:]_]/\1.o/'`; \
 			$(HOSTCC) -M $(HOSTCPPFLAGS) -MQ $(obj)$$g $$f >> $@ ; \
 		done