For use with the jQuery validation plugin.

Just add this javascript code:

jQuery.validator.addMethod(
	"imei",
	function(value, element)
	{
		var numbers = value.split('');
		var sum_string = '', sum = 0;
		for(i = 0; i < numbers.length; i++)
		{
			sum_string += numbers[i] * (i % 2 == 0 ? 1 : 2);
		}
		
		numbers = sum_string.split('');
		for(i = 0; i < numbers.length; i++)
		{
			sum += parseInt(numbers[i]);
		}
		var check = sum % 10 == 0;
		
		return this.optional(element) || check;
	}, 
	"Invalid IMEI number"
);

And add this option to the element you want validated:

$('#form').validate({
	rules: {
		imei: 'imei'
	}
});