How can I add custom conditional validation code to a jQuery event handler? -

i've got jquery in ascx file (sharepoint webpart) responds checkbox's changed event, manipulating properties of control/element based on state of checkbox (checked or unchecked, question):

$(document).on("change", '[id$=ckbxemp]', function () {            var ckd = this.checked;     var $input = $('[id$=txtbxssnoritin]');     if(ckd) $input.data("oldvalue", $input.val()); // remember current value         $input.prop("maxlength", ckd? 4 : 12).css({         background: ckd? 'yellow' : "lightgreen",         width: ckd? 40 : 80     }).val(function(i, v){         // if checked, slice value 4 chars:         return ckd && v.length>4 ? v.slice(0,4) : $input.data("oldvalue");     });     });  

the script above can run this fiddle, based on answer got here

what yet need add custom validation code. if in employee (ssn) mode (the checkbox checked), guess need verify 4 chars in 0..9 subset entered; if itin, though (the checkbox unchecked), need more complicated validation. how incorporate that? need write validation functions , call them "val" function? iow, should line:

return ckd && v.length>4 ? v.slice(0,4) : $input.data("oldvalue"); 

...be changed like:

return ckd && v.length>4 ? getssn($input.data) : getitin($input.data); 

...and code functions so:

function getssn(var entry) {     // validate/verify, slice 4 chars if necessary }  function getitin(var entry) {     // validate/verify, slice 12 if necessary } 

...or what?

update

come think of it, seems logical (in fact, downright obvious) need create event handler textbox, (pseudoscript):

$(document).on("change", '[id$=txtbxssnoritin]', function () {            var $input = $('[id$=txtbxssnoritin]');     var ckbxchecked = "#ckbxemp".checked;     if (ckbxchecked) {         // validate ssn, passing $input     }     else {         // validate itin, passing $input     } }); 

but "how there here" question...