Regex to capture Vietnamese characters (aăâbcdđ..., àáảãạ).
/^[a-z\u0111\u0300\u0301\u0302\u0303\u0306\u0309\u031b\u0323]+$/i
Usage:
regexp.test(str.normalize('NFD'))
Vietnamese characters actually not having 'fjwz', I'm just lazy so I use a-z instead.