I'm on version 5.3.  I reviewed the pase_uri function in the core parse_uri.c but I am not super proficient in C.

case URI_HOST_P:
     switch(*p) {
      check_host_end;
      default:
       if(!isalnum(*p) && (*p != '.') && (*p != '-')
         && !uri_host_char_allowed(*p)) {
        goto error_bad_host;
       }
     }
     break;

I see it calls out periods (.) and dashes (-) but not underscores.  I'm wondering why, being underscores are allowed in URLs.
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