diff --git a/configure.in b/configure.in
index 4a5473b..206b1ff 100644
--- a/configure.in
+++ b/configure.in
@@ -40,7 +40,7 @@
 AM_CONDITIONAL(FLaC__CPU_PPC, test x$cpu_ppc = xtrue)
 AM_CONDITIONAL(FLaC__CPU_SPARC, test x$cpu_sparc = xtrue)
 case "$host" in
-	i[3-6]86-*-openbsd*) OBJ_FORMAT=aoutb ;;
+	i[[3-6]]86-*-openbsd*) OBJ_FORMAT=aoutb ;;
 	*) OBJ_FORMAT=elf ;;
 esac
 AC_SUBST(OBJ_FORMAT)