Javascript 正则表达式 - 重用模式来捕获组

Question

I'm just trying to learn javascript regex and have been stuck on this problem for a while.

I need to match patterns, some examples below:

console.log("42 42 42 42".match(reRegex)); // should NOT match
console.log("42 42".match(reRegex)); // should NOT match
console.log("42 42 42".match(reRegex)); // should match

I have tried a lot of versions of this, but can't figure it out. Can you tell me how this works?

This is incorrect as it matches 42 42 42 42 , which it shouldn't.

let repeatNum = "42 42 42";
let reRegex = /(\d+)(\s)\1\2\1/; // Change this line
let result = reRegex.test(repeatNum);

Thanks

Answer 1

Have alook at anchors

Add a $ if it is per line: /^(\d+)(\s)\1\2\1$/

https://regex101.com/r/FFwPWF/1

Answer 2

You can add ^ at the beginning and add $ at the end of your regex.

https://regex101.com/r/7EUpOb/2

Regex101 is a good reference and has good explanation during execution of regex!

Answer 3

Here is a possible Solution

let repeatNum = "42 42 42";
let reRegex = /^(\d{2,3})(\s)\1\2\1$/; // Change this line
let result = reRegex.test(repeatNum);

It was matching also 42 42 42 42 so you have to limit the regex by adding string anchors at the beginning and end of the regex

Answer 4

/^(\d{2,3})[ ]\1[ ]\1$/

\d{2,3} - should match only 2-3 digit number.
[ ] - Only space character
\1 - captured group 1
^ - beginning of the line.
$ - end of the line

Tested this expression in the link you gave and it passes all tests.

https://www.freecodecamp.org/learn/javascript-algorithms-and-data-structures/regular-expressions/reuse-patterns-using-capture-groups