Hello,

Im try to using the config Core.AllowHostnameUnderscore to allow underscore (_) in the hostname.
Ref: http://htmlpurifier.org/live/configdoc/plain.html#Core.AllowHostnameUnderscore
But it works incorrectly in some cases.

For example:

• This URL will work: https://ex_mple.com/foo/bar
• This URL will not work: http://example_.com/foo/bar
• This URL will not work: http://_sip._udp.apnic.net

Do you have any alternative solution for this issue?

Thank you.