Help understanding the behavior of this regex

JavaScript
1

What is going on here?

Challenge: Regular Expressions - Positive and Negative Lookahead

My problem:
I’ve already “solved” this exercise (i.e. I’ve stumbled upon a solution that passes the test), but I cannot for the life of me figure out why. I’ve googled for more information on how lookaheads function and searched around this forum for examples, but I have been unable to find any answers.

The given criteria:
Use lookaheads in the pwRegex to match passwords that are greater than 5 characters long, and have two consecutive digits.

What I expected to work:

let pwRegex = /(?=.{6})(?=\d{2})/;

What actually works:

let pwRegex = /(?=.{6})(?=\D*\d{2})/;

Without the \D* added, I get these console messages:

Your regex should match the string bana12
Your regex should match the string abc123
Your regex should match the string 8pass99
Your regex should match the string astr1on11aut

I don’t understand how checking for zero or more non-digit characters before the two digits makes the difference, and further experimentation hasn’t yielded any answers, which leads to my question:

What is actually going on here?

Complete code that ‘solves’ the challenge:

let sampleWord = "astronaut";
let pwRegex = /(?=.{6})(?=\D*\d{2})/; // Change this line
let result = pwRegex.test(sampleWord);

My browser information:

User Agent is: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36

2

I find this site very helpful in understanding what’s happening with regex: https://regex101.com/

I think the difference here is using a * (at least 1 match) versus a specific number {2}

(?=.{6})(?=\d*)

This works to find abc123, for example

4

The reason behind the {2} is to ensure the password follows the second criteria: two consecutive digits. It prevents matching of invalid passwords like ‘banan1’.

Also, somebody correct me if I’m wrong, but in this context {2} is functionally the same as {2,}, right? Searching for at least one instance of a sequence of two consecutive digits produces the same result as searching for at least one instance of a sequence of two or more consecutive digits.

5

(?=.{6})(?=.*\d{2})

This works for abc123. I think because the ?= lookahead starts from the beginning each time?

The first section matches 6 characters, but the second section sees “abc123”, it can’t match because the whole sequence “abs123” doesn’t match (?=\d{2}) 2 digits. It does match "any # of char preceding at least 2 digits though (?=.*\d{2})

Asserts that the given subpattern can be matched here, without consuming characters

It matches the string, but doesn’t move the “cursor” forward through the string to the next section.

6

The reason your \D* works is because it matches the “abc” first, then it can match the 2 digits.

7

Too long to explain again. Recycling answer from another post . :slight_smile:

8

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