Username Validation RegExp

Question

Working through the freeCodeCamp JavaScript curriculum and I found this RegExp task surprisingly tricky (maybe I just don't use RegExp very much).

The parameters are:

1. Usernames can only use alpha-numeric characters.
2. The only numbers in the username have to be at the end. There can be zero or more of them at the end. Username cannot start with the number.
3. Username letters can be lowercase and uppercase.
4. Usernames have to be at least two characters long. A two-character username can only use alphabet letters as characters.

My logic is that since usernames must start with a letter we can begin with /^[a-zA-Z]/i.

From there, a username can contain either:

1 or more letters, followed by 0 or more numbers at the end of the string

OR

2 or more numbers at the end of the string.

Which lead me to the following RegExp: /^[a-z]([a-z]+\d*$|\d{2,}$)/i. This RegExp works but I don't write that much RegExp so I'm not confident it's as simple or elegant as it could be.

Answer 1

Your regex looks pretty close to optimal to me, unless I'm missing something. The only adjustments I'd make are:

• Write some tests if you haven't already (I did ad-hoc tests that can run in the browser, but preferably use a real library like Jest or Mocha).
• Use ?: at the start of a parentheses group that you don't need to capture. This communicates the intent of the grouping more clearly to the reader.
• Move $ out of the parens to the end to avoid duplication and improve readability.

const p = /^[a-z](?:[a-z]+\d*|\d{2,})$/i;

const shouldFail = [
  "a",
  "A2",
  "2",
  "2a",
  "a2a",
  "z23a",
  "2B2",
  "AA45678a",
  "",
];
const shouldPass = [
  "aa",
  "aa2",
  "a22",
  "AA23",
  "aa233",
  "zA",
  "AAA",
  "AA45678",
];

const failures = [];

for (const t of shouldFail) {
  if (p.test(t)) {
    failures.push(`'${t}' is invalid but was reported as valid`);
  }
}

for (const t of shouldPass) {
  if (!p.test(t)) {
    failures.push(`'${t}' is valid but was reported as invalid`);
  }
}

failures.forEach(e => console.error(e));

if (failures.length === 0) {
  console.log("All tests passed");
}