Why doesn't this RegExp work as expected?

Question

I made a RegExp to format an incoming date string, but it doesn't work as expected with my usage. I was hoping someone could explain why not:

var data = [
  "m_2013_01_01",
  "m_2013_02_01",
  "m_2013_03_01",
  "m_2013_04_01"
];

// why aren't these equivalent?
//  expected
console.log(data.map(datum => datum.replace(/^m_(\d+)_(\d+)_(\d+)/g, '$1-$2-$3')));
//  ???
console.log(data.map(datum => datum.replace(/^m_(?:(\d+)_?){3}$/g, '$1-$2-$3')));

You should never use quantifiers on capturing groups, if you actually want to use them in any way, as only the last occurence of this group is actually stored. — Sebastian Proske, Jul 08 '16 at 21:22

score 5 · Accepted Answer · answered Jul 08 '16 at 21:24

5

In the fist regex you are using 3 groups:

Regular expression visualization

That's why you can reference group 1, 2 and 3.

However, in the second regex you are using 1 group repeated multiple time, so group 2 and 3 doesn't exist and can't be referenced:

Regular expression visualization

answered Jul 08 '16 at 21:24

Federico Piazza

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Whoa! Where did you get these control flows? – Patrick Roberts Jul 08 '16 at 21:25
@FedericoPiazza Actually Debuggex has a share feature specifically for Stack Overflow which also links to Debuggex. – Sebastian Simon Jul 08 '16 at 21:26
@Xufox, yes, I didn't want to paste the full debuggex share feature. The image was just ilustrative to answer the question – Federico Piazza Jul 08 '16 at 21:27
@PatrickRoberts, also... keep in mind that you can debug step by step the regex by using the slider ![Regular expression visualization](https://www.debuggex.com/i/LBmrtfB1CfRVnLoZ.png) – Federico Piazza Jul 08 '16 at 21:36

Why doesn't this RegExp work as expected?

1 Answers1