Capturing groups in Javascript Regex not populating back-references correctly

Question

Capturing groups in Javascript Regex not populating back-references correctly

Here's an interesting situation (or maybe not so uncommon): I'm trying to extract two specific capturing groups using JavaScript regex. The first group should consist of one or more digits (0-9), while the second group should consist of one or more word characters or hyphens (A-Z, 0-9, -), but for some reason, I can never seem to retrieve the latter group.

Important note: I have intentionally used the alternation (|) character as I want to potentially match either one or the other

This is the code snippet I am currently working with:

var subject = '#/34/test-data'
var myregexp = /#\/(\d+)|\/([\w-]+)/;
var match = myregexp.exec(subject);
if (match != null && match.length > 1) {
  console.log(match[1]); // successfully returns '34'
  console.log(match[2]); // why does this return undefined instead of 'test-data'?
}

The strange thing is that Regex Buddy confirms the presence of two capturing groups and even highlights them correctly in the test phrase.

Could this issue be related to a problem in my JavaScript syntax?

javascript regex capturing-group

Answer 1

Answer №1

If you make a modification:

var myregexp = /#\/(\d+)\/([\w-]+)/;

by eliminating the | alternation meta-character to become:

var myregexp = /#\/(\d+)\/([\w-]+)/;

it will match both groups. Currently, your regex is searching for either \d+ or [\w-]+, so once it finds the first group it stops and the second one remains empty. By removing |, it now looks for \d+ followed by /, then by [\w-]+, ensuring it matches both or neither.

Edit: To match all of #/34/test-data, #/test-data or #/34, you can use #(?:\/(\d+))?\/([\w-]+) instead.

Answer 2

If you make a modification:

var myregexp = /#\/(\d+)\/([\w-]+)/;

by eliminating the | alternation meta-character to become:

var myregexp = /#\/(\d+)\/([\w-]+)/;

it will match both groups. Currently, your regex is searching for either \d+ or [\w-]+, so once it finds the first group it stops and the second one remains empty. By removing |, it now looks for \d+ followed by /, then by [\w-]+, ensuring it matches both or neither.

Edit: To match all of #/34/test-data, #/test-data or #/34, you can use #(?:\/(\d+))?\/([\w-]+) instead.

Answer 3

Answer №2

By eliminating the "|" from your code, you will achieve the desired outcome. Does this explanation clarify things for you?

let subject = '#/34/test-data'
let myregexp = /#\/(\d+)\/([\w-]+)/;
let match = myregexp.exec(subject);
if (match !== null && match.length > 1) {
  console.log(match[1]); // successfully returns '34'
  console.log(match[2]); // Why is it 'undefined'? It should be 'test-data'
}

Keep up the good work!

Update:

I believe the issue you encountered stemmed from using the "|", which instructed JS to capture either the first group or the second one. Due to JS's lazy evaluation, it stopped at the first group. By excluding the OR operator from the RegExp pattern, you can obtain both results effectively (similar to an AND operation).

Answer 4

By eliminating the "|" from your code, you will achieve the desired outcome. Does this explanation clarify things for you?

let subject = '#/34/test-data'
let myregexp = /#\/(\d+)\/([\w-]+)/;
let match = myregexp.exec(subject);
if (match !== null && match.length > 1) {
  console.log(match[1]); // successfully returns '34'
  console.log(match[2]); // Why is it 'undefined'? It should be 'test-data'
}

Keep up the good work!

Update:

I believe the issue you encountered stemmed from using the "|", which instructed JS to capture either the first group or the second one. Due to JS's lazy evaluation, it stopped at the first group. By excluding the OR operator from the RegExp pattern, you can obtain both results effectively (similar to an AND operation).

Capturing groups in Javascript Regex not populating back-references correctly

Answer №1

Answer №2

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