正则表达式实现科学计数法

    const str = '100000000000000'
    const reg = /(?=(\d{3})+$)/g
    console.log(str.replace(reg, '.'))  //   .100.000.000.000.000

    const str = '100000000000000'
    const reg = /(?=(\B)(\d{3})+$)/g
    console.log(str.replace(reg, '.'))  //   100.000.000.000.000