"12345678901890".replace(/(\d)(?=(\d{3})+$)/g,'$1,')分析过程：(\d)匹配到1(?=(\d{3})+$)匹配到2345678901890末尾的0不匹配到((\d{3})+$)(\d)匹配到2(?=(\d{3})+$)匹配到345678901890ending890匹配到((\d{3})+$)(\d)匹配到3(?=(\d{3})+$)匹配到45678901890在90的末尾不匹配((\d{3})+$)...所以匹配是12,345,678,901,890
