i got this regular expression in password validation, it works well, but i really do not understand it's meaning. it just lookahead with nothing before it ?

const PwdReg = /(?!^[0-9]+$)(?!^[a-z]+$)(?!^[A-Z]+$)(?!^[^A-z0-9]+$)^[^\s\u4e00-\u9fa5]{6,12}$/