regex to replace multiple subsequent line breaks

Question

Here is what I want to do.

Replace more than 3 line breaks with 3 line breaks.
Replace 2 line breaks with 1 line break.
Ignore single line breaks.

So that something like this:

Dear Bob



I would love to not have so many line breaks here.

This is a new paragraph.




Thanks

Jim

Ends up more like this:

Dear Bob

I would love to not have so many line breaks here.
This is a new paragraph.

Thanks
Jim

Based on another question, this is the closest I've come, but it's not quite right:

innerHTML.replace(/\n\n\s*\n\n/g, '\n');

Not quite right how? Remember "more than three" means \n\n\n+. — tadman
– tadman, Commented Mar 29, 2016 at 21:30
It's hard to explain because the real data is private. I will try to obfuscate it and add the exact results to my question as soon as I can. — user736893
– user736893, Commented Mar 29, 2016 at 21:31
If you can create some dummy text in a similar structure that would be sufficient. — tadman
– tadman, Commented Mar 29, 2016 at 21:37

Wiktor Stribiżew · Accepted Answer · 2016-03-29 21:46:17Z

1

You may use a regex with an alternation group, one alternative will match 4+ linebreaks, and the other just 2 (that is not preceded nor followed with a line break).

The regex will be:

((?:\r?\n){4,})|(^|[^\n])(?:\r?\n){2}(?!\r?\n)

Explanation:

((?:\r?\n){4,}) - Alternative 1 matching 4+ sequences of an optional \r followed with a compulsory \n
| - or...
(^|[^\n])(?:\r?\n){2}(?!\r?\n) - Alternative 2 matching exactly 2 sequences of an optional \r followed with a compulsory \n that are not preceded ((^|[^\n]) matches either the start of string or a character other than \n) nor followed with a linebreak (the negative lookahead (?!\r?\n) makes sure of that).

In the replacement, there is a callback checking which alternative matched and replaces accodingly.

The JS code demo is below:

var re = /((?:\r?\n){4,})|(^|[^\n])(?:\r?\n){2}(?!\r?\n)/g; 
var str = `Dear Bob



I would love to not have so many line breaks here.

This is a new paragraph.




Thanks

Jim`;
var result = str.replace(re, function (m, g1, g2) {
	return g1 ? "\n\n" : g2 + "\n";
});
document.body.innerHTML = "<pre>" + result + "</pre>";

answered Mar 29, 2016 at 21:46

Wiktor Stribiżew

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3 Comments

Wiktor Stribiżew Over a year ago

You can adjust the linebreaks, e.g. add \r.

user736893 Over a year ago

If this were Reddit I would tag you as Regex God

Wiktor Stribiżew Over a year ago

I am not a regex god. :)

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