1.将数字中间几位以*号代替，前后各保留 4 位

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var string = "1234567894";
var s = string.replace(/^(.{4})(?:\d+)(.{4})$/, "$1***$2");
console.log(s); // 1234***7894

2.字符串驼峰转下划线

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var string = 'isGood'
var s = string.replace(/([A-Z])/g, '_$1').toLowerCase()
console..log(s) // is_good