diff --git a/configure.ac b/configure.ac
index 4f00ff01..55600d90 100644
--- a/configure.ac
+++ b/configure.ac
@@ -275,7 +275,7 @@ test "$prefix" = NONE && prefix=$ac_default_prefix
 test "$exec_prefix" = NONE && exec_prefix='${prefix}'
 for name in $ac_subst_vars; do
     for _ in 1 2 3; do
-    if printf '%s' "${name}" | egrep '^[_[:alpha:]][_[:alnum:]]*$'
+    if printf '%s' "${name}" | egrep '^[[_[[:alpha:]]]][[_[[:alnum:]]]]*$'
     then
         eval "${name}"="$(eval echo "\${${name}}")"
     fi