How to regex replace a repeated sequence?

Question

For example, if there a sequence of 2 or more :0000: then I like to replace them with :.

In the code below, the result should be 2a00:1450:4028:805::00001111:2005 but it just ignores the replacement.

BTW, if it's just a single :0000: then it should be replaced with :0:, but this should be fairly easy once the real issue is solved. Note I've deliberately used 00001111 to make sure it doesn't get replaced.

let str = document.getElementById("input").textContent; 
let res = str.replace(/(:0000:){2,}/g, ":");
document.getElementById("output").textContent = res;

Before:
<p id="input">
2A00:1450:4028:080B:0000:0000:00001111:2005
</p>
After
<p id="output">
</p>

No, I've updated now to reflect why it's deliberately like this. — LWC
– LWC, Commented Nov 8, 2024 at 22:47
of course, even a correct regex will also need to find the longest repeated sequence of :0000: - e.g. 1234:0000:0000:5678:0000:0000:0000:1234 would not become 1234::5678::1234 nor 1234::5678:0000:0000:0000:1234 ... it would be 1234:0000:0000:5678::1234 - so, since you need to find the longest sequence of 0000's, you'll probably end up with code that won't need to use regular expressions anyway — Jaromanda X
– Jaromanda X, Commented Nov 8, 2024 at 23:13
^^ I am assuming you are dealing with "compressing" IPv6 addresses — Jaromanda X
– Jaromanda X, Commented Nov 8, 2024 at 23:21
What's the expected result if your string starts with the pattern like?0000:0000:2A00..., is it ::2A000... — Nick Parsons
– Nick Parsons, Commented Nov 9, 2024 at 6:22

Barmar · Accepted Answer · 2024-11-08 23:08:14Z

1

The problem is that your pattern /(:0000:){2,} requires that each 0000 be separated by ::, not just :, e.g. :0000::0000::0000:. You should have : at the beginning or end of the group, not both. Then you can match another : outside the quantified group.

And if you want :: in the result, you should use that in the replacement, not just :.

let str = document.getElementById("input").textContent; 
let res = str.replace(/(:0000){2,}:/g, "::");
document.getElementById("output").textContent = res;

Before:
<p id="input">
2A00:1450:4028:080B:0000:0000:00001111:2005
</p>
After
<p id="output">
</p>

answered Nov 8, 2024 at 23:08

Barmar

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