diff --git a/t/t0110-urlmatch-normalization.sh b/t/t0110-urlmatch-normalization.sh
index f99529d838..4dc9fecf72 100755
--- a/t/t0110-urlmatch-normalization.sh
+++ b/t/t0110-urlmatch-normalization.sh
@@ -47,7 +47,7 @@ test_expect_success 'url authority' '
 	test-tool urlmatch-normalization "scheme://@host" &&
 	test-tool urlmatch-normalization "scheme://%00@host" &&
 	! test-tool urlmatch-normalization "scheme://%%@host" &&
-	! test-tool urlmatch-normalization "scheme://host_" &&
+	test-tool urlmatch-normalization "scheme://host_" &&
 	test-tool urlmatch-normalization "scheme://user:pass@host/" &&
 	test-tool urlmatch-normalization "scheme://@host/" &&
 	test-tool urlmatch-normalization "scheme://host/" &&
diff --git a/urlmatch.c b/urlmatch.c
index 33a2ccd306..03ad3f30a9 100644
--- a/urlmatch.c
+++ b/urlmatch.c
@@ -5,7 +5,7 @@
 #define URL_DIGIT "0123456789"
 #define URL_ALPHADIGIT URL_ALPHA URL_DIGIT
 #define URL_SCHEME_CHARS URL_ALPHADIGIT "+.-"
-#define URL_HOST_CHARS URL_ALPHADIGIT ".-[:]" /* IPv6 literals need [:] */
+#define URL_HOST_CHARS URL_ALPHADIGIT ".-_[:]" /* IPv6 literals need [:] */
 #define URL_UNSAFE_CHARS " <>\"%{}|\\^`" /* plus 0x00-0x1F,0x7F-0xFF */
 #define URL_GEN_RESERVED ":/?#[]@"
 #define URL_SUB_RESERVED "!$&'()*+,;="