Could you point what must to be adjusted to make this textbox to accept only letters and numbers (regex), only when it's filled?

Hi Experts

Could you point what must to be adjusted to make this textbox to accept only letters and numbers (regex), only when it's filled - no required field?

<span class="validar">
	<input type="text" name="sise" id="sise" class="form-control gui-input br-light light" placeholder="">
</span>

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It doesn't accept empty textbox

Correctly applies regex:


jQuery code:

$.validator.addMethod('regex', function (value, element, regexp) {
	var pattern = new RegExp(regexp);
	return pattern.test(value);
}, 'Use o formato correto'); // Use correct format


$('#entity-form').on('submit', function (e) {
	e.preventDefault();
}).validate({

	ignore: ".ignore",
	errorClass: "state-error",
	validClass: "state-success",
	errorElement: "em",
	rules: {
		fk_seguradora: {
			required: true,
		},
		data: {
			required: true,
		},
		sise: {
			
			// regex and validation applied
			regex:/^[A-Za-z0-9]*\d+[A-Za-z0-9]*$/
			required: false, // This causes error - page doesn't run!!!
			//--------------------------------------
		},
	},
	messages: {
		nome_reclamante: {
			required: '',
		},
		
	//--------------------------------------------------	
	//Here: Must to accept only leters and numbers
		sise: {
			//-- How to adjust here(???)
			number: '',
		},
	 //---------------------------------------------
	},
	highlight: function (element, errorClass, validClass) {

		$(element).closest('.validar').addClass(errorClass).removeClass(validClass);
	},
	unhighlight: function (element, errorClass, validClass) {
		$(element).closest('.validar').removeClass(errorClass).addClass(validClass);
	},
});

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Thanks in advance!
img_EE_141216_001.png