Question about Regular Expressions: Positive and Negative Lookahead

JavaScript
1

Hi everyone.

I’m trying to understand how lookaheads work, so I tested the following code, based on the challenge on topic:

let sampleWord1 = "astrona88ut"; 

let sampleWord2 = "astron88aut";

let testRegex = /(?=\w{5,})(?=\d{2})/;

console.log(testRegex.test(sampleWord1));   // Returns False

console.log(testRegex.test(sampleWord2));    //Returns True

Can anyone explain to me why I got False and True, respectively?

Thanks!

2

Yes but it isn’t easy =)

The regex /(?=\w{5,})(?=\d{2})/ is a single expression, not two separate ones. It also is very literal in that you are telling it to find at least 5 characters that begin with two digits.

That’s why this regex /(?=\w{5,})(?=\D*\d{2})/ returns true for sampleWord1. It looks for 5 characters that start with at least one non digit followed by two digits.

I hope that’s somewhat clear.
-J

3

Hi Jesse. Thank you for your answer.

The regex /(?=\w{5,})(?=\d{2})/ is a single expression, not two separate ones. It also is very literal in that you are telling it to find at least 5 characters that begin with two digits.

I think I understand what you mean. But if that is the case, why did console.log(testRegex.test(sampleWord2)) return True on my code? The two digits were not at the beginning, but in the middle of the word ( let sampleWord2 = "astron88aut";).

4

Because there is an instance of a 5-character set that starts with 2 digits “88aut”. In sampleWord1 it was “88ut” which fails the test.

/(?=\w{5,})/ //Is there a set of at least 5 characters?

/(?=\d{2})/ //Is there a set of exactly two digits?

/(?=\w{5,})(?=\d{2})/ //Is there a set of at least 5 characters that starts with exactly two digits?

/(?=\w{5,})(?=\D*\d{2})/ //Is there a set of at least 5 characters that contains exactly two digits and may or may not start with one or more non-digits?
-J

2 Likes
5

Got it! Your explanation was superb now. I was losing my mind over this, lol.

Thank you very much!!

1 Like