Specify Exact Number of Matches

JavaScript
1

Tell us what’s happening:
What am i missing? i match for exactly 4, it passes on RegExr, why is this throwing error for more than 4 m’s?
Error: Your regex should not match “Timber” with 30 m’s in it.

Your code so far


let timStr = "Timmmmber";
let timRegex = /m{4}/; // Change this line
let result = timRegex.test(timStr);

Your browser information:

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.181 Safari/537.36.

Link to the challenge:
https://learn.freecodecamp.org/javascript-algorithms-and-data-structures/regular-expressions/specify-exact-number-of-matches/

2

Because you didn’t account for repeating characters. In essence your regex says, give me exactly 4 m’s consecutively. Which leads to this issue

Ti [mmmm][mmmm][mmmm][…]ber.

That matches groups of 4 m’s. I know, the docs aren’t really clear on that.

You need to qualify the regex to return no more than 4 m’s.

1 Like
3

Thanks… i’m searching the internet high and low for any reference to this. The lesson and examples are regarding the {4} notation. And it works as expected in several regex tools. If anyone has a link to how to do ‘no more than’… even though {4"} supposed to be exactly :rofl: Any direction would be greatly appreciated.

4

I’ll give you one last hint and a spoiler to my solution. Look into these:

  • positive lookahead
    • Matches a group after the main expression without including it in the result.
  • negative lookahead
    • Specifies a group that can not match after the main expression (if it matches, the result is discarded).
  • negated set
    • Match any character that is not in the set.

Solution:

let timRegex = /[^m]m{4}(?!m)/

// [^m]  is a negated set. It will only return a match if the 
//       letter 'm' is not the first character in the entire regex match

// m{4}  matches exactly 4 m's

// (?!m) is a negative lookahead that returns a match 
//       only if the matched text before it is not followed by an 'm'
5

Interesting solution JM. I think this challenge should be further down the line in the section, since the lookahead challenges were ahead of this one (making it hard to figure out that it should be used here).

2 Likes
6

This simpler solution also works:

let timRegex = /Tim{4}ber/;
4 Likes
7

let timStr = “Timmmmber”;

let timRegex = /Tim{4}ber/; // Change this line <==

let result = timRegex.test(timStr);