正则表达式以匹配特定位数

Question

I am trying to write a regular expression which returns a three digit integer only. Not less than three or more than three. However my regex below is also true for four digit numbers. Am i missing something?

var threeDigits = /\d{3}$/

console.log(threeDigits.test("12"))// false
console.log(threeDigits.test("123"))// true
console.log(threeDigits.test("1234"))// true yet this is four digits???

Answer 1

You have the ending anchor $ , but not the starting anchor ^ :

var threeDigits = /^\d{3}$/

Without the anchor, the match can start anywhere in the string, eg

"1234".match(/\d{3}$/g)  // ["234"]

Answer 2

使用一个^ [0-9] {3} $或^ \\ d {3} $。