diff --git a/tests/Makefile b/tests/Makefile
index 11164e9e..8ce8f209 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -5,7 +5,7 @@
 TOPDIR = ..
 include $(TOPDIR)/include/builddefs
 
-TESTS_SUBDIRS = $(sort $(dir $(wildcard $(CURDIR)/[a-z]*/)))
+TESTS_SUBDIRS = $(sort $(dir $(wildcard $(CURDIR)/[[:lower:]]*/)))
 
 include $(BUILDRULES)