What is wrong in my regex /(?=^[a-z]+\d{2,})(?=\w{5,})/ pattern?

Question

This regex has to match passwords that are greater than 5 characters long, do not begin with numbers, and have two consecutive digits.

All the test cases are passing the regex test.

My regex is /(?=^[az]+\\d{2,})(?=\\w{5,})/

I have to use two positive lookaheads to solve this problem to pass the tests.

But astr1on11aut is not passing the test. Why?

Link to problem- https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/regular-expressions/positive-and-negative-lookahead

Answer 1

Your regex fails because of the first lookahead pattern (?=^[az]+\\d{2,}) . The string "astr1on11aut" starts with lowercase letters:

astr1on11aut
^^^^

This matches ^[az]+ . However, the next part of the pattern demands two or more digits with \\d{2,} , however the string only has one at that place:

   astr1on11aut
       ^^
       ||
digit -+|
        + --- not a digit

This causes the first lookahead pattern to fail.

You can express the validation rules more cleanly with three lookaheads:

• "greater than 5 characters long: (?=.{5,})
• "do not begin with numbers": ^(?!\\d)
• "and have two consecutive digits": (?=.*\\d{2})

If we put them all together we get /(?=.{5,})(?!^\\d)(?=.*\\d{2})/

 const regex = /^(?=.{5,})(?!\\d)(?=.*\\d{2})/; test("abc"); test("123"); test("123abc"); test("abc123"); test("astr1on11aut"); test("., ;_'@=-%"); test("., ;_'@123=-%"); function test(string) { console.log(`${string} : ${regex.test(string)}`); }

Note that this regex doesn't require letters . Strictly following the requirements, the only thing explicitly asked for is digits. Since the type of any other input is not specified, it's left to be anything (using . ). It's best not to make too many assumptions when writing a regular expression or you might block legitimate input.

Answer 2

If you are not limited to using a single regex, I suggest splitting this into multiple tests in your host language (eg JavaScript):

if (input.match(/^\D/)
    && input.match(/\d{2}/)
    && input.length >= 5) {
  // password policy fulfilled
}

Answer 3

Does this work for you?

(?=^\D.{5,}$).*(?=\d{2,})

The first lookahead asserts that the string must not begin with a digit but be at least 6 chars long; the second asserts that there must be at least 2 consecutive digits.

Answer 4

This regexp passes all the tests

/(?=\w*\d\d)(?=\w{5,})(?=^[^0-9]\w*)/

I believe you could fix yours by splitting the first group.