Easiest way: add an extra letter at the end of the line. Select that letter. Hit enter. The letter will be replaced with a new line.

When you're focused on the cell press enter to get into edit mode:

• Windows/Linux/Chrome OS: Ctrl + Enter
• Mac: Command/option(Alt) + Enter

Here is what I did.

1. Hit ctrl+f to search, and click the "..." button representing more options

   

2. Typed \s+$ or \n in the find field

3. I left the replace field empty
4. Clicked "Search Using RegEx"
5. Clicked replace

I hope it works for you. Also, you can replace \n with any regex expression and it can be immensely helpful.

IF you are trying to use the Ctrl+F to find and replace well you can't. at least I dont know how.

but if you have a cell say A1 that contains the following : "stack,overflow" then if you type this in A2 it should work: =SUBSTITUTE(A1, ",", char(10))

https://support.google.com/docs/bin/static.py?hl=en&topic=25273&page=table.cs

you can then copy the content of A2 and in A3 right click -> Paste Special... -> Values Only.

Hope this helps

UPDATE 2016-AUG Google Spreadsheet now support searching for newline using \n here is how: Edit -> Find and Replace

How about following sample? This sample supposes that the line break is \r\n and \n.

FLow :

1. Retrieve the information of line break.
2. If the line break is \r\n, when the number of \r\n is more than 2, it is modified to char(10).

Sample :

=IF(REGEXMATCH(E1, "\r\n"),REGEXREPLACE(E1, "(\r\n){2,}", char(10)),REGEXREPLACE(E1, "(\n){2,}", char(10)))

Result :

If you want to remove all of the line break, you can use =CLEAN(A1). In this case, the result of the result sheet becomes sample1sample2sample3sample4sample5.

If this was not helpful for you, I'm sorry.

char(10) is for newline, so:

=SUBSTITUTE(A1,char(10),", ")