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Order of Operations in Regular Expressions (solving a challenge)

I am working on a problem from FreeCodeCamp that involves me developing a Regular Expression that meets these four requirements:

Usernames can only use alpha-numeric characters.
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.
Username letters can be lowercase and uppercase.
Usernames have to be at least two characters long. A two-character username can only use alphabet letters as characters.

What I have decided on is /[a-z]/ig, which covers all of the test cases except for the following three:

Your regex should not match the string BadUs3rnam3
Your regex should match the string Z97
Your regex should not match the string c57bT3

What confuses me is that if I add numbers (\d) into my Regex then some of my simpler requirements ("Regex should match JACK") are now incorrect.

I was thinking that I could do something like:

/[a-b+?]*\d/

or this

/[a-b^0-9$]/

But that throws off things.

Can someone lend me a hand and explain how this might work?

about 4 years ago · Juan Pablo Isaza

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Answer 1 · 2022-08-31T10:16:50.704Z

The patterns that you tried do not match because:

The pattern [a-b+?]*\d matches optional chars a b + or ? followed by a single digit, which makes the digit mandatory and matches not allowed chars
The pattern [a-b^0-9$] only matches a single char a b ^ digit 0-9 or $ and also matches not allowed chars

You could either match 2 or more chars a-z followed by optional digits, or match a single char a-z followed by 2 or more digits.

^[a-z](?:[a-z]+\d*|\d{2,})$

Explanation

^ Start of string
[a-z] Always start with a char a-z
(?: Non capture group
- [a-z]+\d* Match 1 or more chars a-z and optional digits
- | Or
- \d{2,} Match 2 or more digits
) Close non capture group
$ End of string

Regex demo

const regex = /^[a-z](?:[a-z]+\d*|\d{2,})$/i;
[
  "aa",
  "JACK",
  "abc",
  "ab1",
  "Z97",
  "BadUs3rnam3",
  "c57bT3",
  "a",
  "0",
  "01",
  "a1",
  "a",
  "ab1a"
].forEach(s => console.log(`${s} --> ${regex.test(s)}`));

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Order of Operations in Regular Expressions (solving a challenge)

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