ABN - Australian Business Numbers I knew they had to be numeric and 11 digits. Then i found this: http://www.ato.gov.au/businesses/content.asp?doc=/content/13187.htm&pc=001/003/021/002/001&mnu=610&mfp=001/003&st=&cy=1
Which explains quite nicely how to validate a number. So I thought I would share this: function abnValidate(value, element){ if (value.length != 11 || isNaN(parseInt(value))) return false; var weighting = [10,1,3,5,7,9,11,13,15,17,19]; var tally = (parseInt(value[0]) - 1) * weighting[0]; for (var i = 1; i < value.length; i++){ tally += (parseInt(value[i]) * weighting[i]); } return (tally % 89) == 0; } jQuery.validator.addMethod( 'abnValidate', abnValidate, 'This ABN is not valid' );