restrict-possible-usernames.txt

let username = "JackOfAllTrades";
let userCheck = /^[a-z][a-z]+\d*$|^[a-z]\d\d+$/i;
let result = userCheck.test(username);

//comment: 1) ^ - start of input 2) [a-z] - first character is a letter 3) [a-z]+ - following characters are letters 4) \d*$ - input ends with 0 or more digits 5) | - or ^[a-z] - first character is a letter 6) \d\d+ - following characters are 2 or more digits 7) $ - end of input 

console.log(result)