lib/PublicInbox/AltId.pm  | 2 +-
 lib/PublicInbox/Search.pm | 2 ++

diff --git a/lib/PublicInbox/AltId.pm b/lib/PublicInbox/AltId.pm
index 8ce70e46042f0b7e1c38e2fe191ae2a58eb548a7..3be6c73cc0e93c52220e554543bc640e7123f1a8 100644
--- a/lib/PublicInbox/AltId.pm
+++ b/lib/PublicInbox/AltId.pm
@@ -22,7 +22,7 @@ sub new {
 	my ($class, $ibx, $spec, $writable) = @_;
 	my ($type, $prefix, $query) = split(/:/, $spec, 3);
 	$type eq 'serial' or die "non-serial not supported, yet\n";
-
+	$prefix =~ /\A\w+\z/ or warn "non-word prefix not searchable\n";
 	my %params = map {
 		my ($k, $v) = split(/=/, uri_unescape($_), 2);
 		$v = '' unless defined $v;
diff --git a/lib/PublicInbox/Search.pm b/lib/PublicInbox/Search.pm
index fabf5301f163187035c2cdd1a51895e98ba8116a..86a6ad674b3e1fbfdb70d3a04d8dac40d4fae881 100644
--- a/lib/PublicInbox/Search.pm
+++ b/lib/PublicInbox/Search.pm
@@ -316,6 +316,8 @@ 	if (my $altid = $self->{altid}) {
 		my $user_pfx = $self->{-user_pfx} = [];
 		for (@$altid) {
 			# $_ = 'serial:gmane:/path/to/gmane.msgmap.sqlite3'
+			# note: Xapian supports multibyte UTF-8, /^[0-9]+$/,
+			# and '_' with prefixes matching \w+
 			/\Aserial:(\w+):/ or next;
 			my $pfx = $1;
 			push @$user_pfx, "$pfx:", <<EOF;