Regex - replace multiple line breaks with two line break and remove line break if at the end?

Question

I'm doing this (+ JSON.stringify()) for sending linebreaks via ajax to my mysql server:

var newChatComment = varChatComment.replace(/\r\n|\r|\n/g,'\n');

How can I also replace multiple line breaks with two line break and in case of if the line break(s) is(are) at the end ... remove the line break?

• plus in case of only one line break ... keep this line break at one?

Edit: The duplicate example pointed to doesn't consider the fact to remove the line break in case the line break is at the end. So this is a new question

Answer 1

If you need to keep a single line break at the start of the string, use

 var varChatComment = "\\r\\n\\n1\\r\\n2\\n\\n\\n3\\r\\r\\n4\\r\\n\\r\\n"; var newChatComment = varChatComment.replace(/((?:\\r\\n?|\\n)+)$|(?:\\r\\n?|\\n){2,}/g, function ($0,$1) { return $1 ? '' : '\\n\\n'; }); console.log(newChatComment); 

Note that the pattern is built around a (?:\\r\\n?|\\n) construct that matches:

• \\r\\n? - a CR and an optional LF
• | - or
• \\n - an LF symbol.

Details :

• ((?:\\r\\n?|\\n)+)$ - 1 or more line breaks at the end of the string (those will be removed)
• | - or
• (?:\\r\\n?|\\n){2,} - 2 or more line breaks (any style) (this will be turned into 2 newlines.

The ((?:\\r\\n?|\\n)+)$| alternative matches any line break(s) at the end of the string, and if found, these ones are replaced with the empty string.