JavaScript Simple Validation

I need two simple validations in javascript. First, check if the first field of an input is a letter, and if it contains numbers, at most 8: Valid field example: J2191023 A0192301 Exemplo de campo INVALIDO: J1234567 J0123456 J1231234 812391JC 123121JA If using strings, 1234567 111111 222222 333333 after the letter, invalidate! Second, check if the second field (PASSWORD FIELD) of an input contains letters and numbers, also at most 8, Example of valid second field: 30manu30 joao2102 24cesaru 1jota91s Invalidate the field if there are only numbers or only letters, the validation must be alphanumeric!