To find paragraph breaks (i.e. the ASCII line feed character 0A hex) in LibreOffice, use the currency symbol “$” as the search string.
Make sure to look into “Other options” and check “Regular expressions” in the “Find & Replace” dialog.
By the way, searching for “\n” and replacing it with “\n” can be done only once, as on the repeated action there will be no matches found anymore and searching for “$” must be done instead. Read some more about this bug/feature/inconsistency here.
You may experience a situation where may not be able to reset filters in LibreOffice, with the Data -> More filters -> Reset all.
Use the alternative way by directly ordering the rows to be shown by Format -> Row -> Show.
Let’s say you have a list of e-mails in a column in LibreOffice Calc spreadsheet and you want to see if there are cells which contain more than one e-mail address.
To do that you can use the formula below (assuming your e-mails are stored in column A) which will show the number of symbols @ in the particular cell: