题解 | #判断是否符合 USD 格式#

 let flag = false
    let arr = str.split('.')[0]
    let arrIs = str.split('.')[1] || []
    let array = arr.split(',')
    for (let i = 0; i < array.length; i++) {
        let str1 = array[i]
        if (i === 0 && str1.length <= 4 && str1.length !== 1 && str1[0] === '$') {
            flag = true
        } else if (str1.length !== 3 ) {
            flag = false
        } 

    }
    if (arrIs.length === 0 && flag) {
        flag = true
    } else if (arrIs.length !== 2) flag = false
    return flag