To get a new paragraph symbol (Enter), type
\n in the Replace: field, and check Regular expressions.
Maybe your Find: and Replace: fields includes a point (
.); you need to escape it with a backslash (
See more LibreOffice Help on Regular Expressions. The
$ symbol could be of interest in this case.
Tested with LibreOffice 22.214.171.124 (x67); OS: Windows 10.0.
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