lib/PublicInbox/WwwAttach.pm | 2 +-

diff --git a/lib/PublicInbox/WwwAttach.pm b/lib/PublicInbox/WwwAttach.pm
index d690ce411921b5fa40820b98923b53d23c1f001a..96103cb028e09b014e4bf29fb7e711c4e567ec08 100644
--- a/lib/PublicInbox/WwwAttach.pm
+++ b/lib/PublicInbox/WwwAttach.pm
@@ -27,7 +27,7 @@ 		# discrete == type, we remain Debian wheezy-compatible
 		if ($ct && (($ct->{discrete} || '') eq 'text')) {
 			# display all text as text/plain:
 			my $cset = $ct->{attributes}->{charset};
-			if ($cset && ($cset =~ /\A[\w-]+\z/)) {
+			if ($cset && ($cset =~ /\A[a-zA-Z0-9_\-]+\z/)) {
 				$res->[1]->[1] .= qq(; charset=$cset);
 			}
 		} else { # TODO: allow user to configure safe types