Check the validity of the format of an EU VAT number. jsvat is a small library to check validity of European (and few non-eu) VAT number. (learn more about VAT) jsvat use 2-step check (see below) and didn't make any request for external resources. Each country has own regexp for VAT number and different math-logic of number calculating. There is 2-step check: Compare with list of Regexps; For example regexp for austria is /^(AT)U(\d{8})$/. Looks like ATU99999999 is valid (it's successfied the regexp), but actually it's should be invalid. Some magic mathematical counting; Here we make some mathematical calculation (different for each country). After that we may be sure that ATU99999999and for example ATV66889218 isn't valid, but ATU12011204 is valid.