title: How does a regex match the format like xxxx1111 in js?
body: <p>I'm limiting the user to input a string contains four letters following with 4 digit of numbers, like the format qwer1234 in vue.js. I have tried the regex <code>b[a-z]{4}+[0-9]{4}\b</code> but it seems not working. Are there any mistake I make?</p>