[Aetae]

/^[a-z]\w*$/i

var reg = /^([a-z])([a-z\d_]+)/i;
['vasea', '_ssa', '343sasassa'].forEach(function(value){
alert(reg.test(value));
});

var reg = /^([a-z])([a-z\d_]+)/i;
alert(reg.test('vasea-лох'));

=)