[jQuery] Alphanumeric plugin doesn't prevent underscores

[BSpizzle]

something I have noticed and don't seem to be able to find a way
around is that with the alphanumeric plugin (http://itgroup.com.ph/
alphanumeric/) and in all demos I've seen online and in my use of
this, it appears that the underscore character is not parsed out like
every other character. the 'ichars' group should allow me to add
custom exception characters but doesn't... am I missing something
obvious here?

here is an excerpt from the plugin (unpacked) with the underscore
added (it isn't there standard):
.................................................................
$.fn.alphanumeric = function(p) {

                p = $.extend({
                        ichars: "!...@#$%^&*()+=[]\\\';,/{}|\":<>?~`.-_ ",
                        nchars: "",
                        allow: ""
                  }, p);
.................................................................


please help!