Keep what's there without the non-printing chars

The standard ‘F&R’ does not accept the search expression $$.
Only the single $ will work for the purpose. You may, of course, use an intemediary placeholder to prepare for the second step.

No, I don’t want to edit my post. In it, I tried to address your initial requirement, and in edit, I addressed the attachment you’ve added. So, there’s no “confusing phrases”: the file definitely contains line breaks, not paragraph breaks, and my edit fully covers the file’s case. If the file is irrelevant to question, then it’s the file that confuses, not my phrases.

Ah! I see. The file isn’t ODF; actually, it’s plain text that contains LFs, and they would be treated differently depending on LO settings (and they in turn depend on OS by default); that’s why it opens as a document with line breaks. So, please don’t assign arbitrary extensions to files regardless of their format; this specific file needs a TXT extension.

For this file, if you open it with formatted text and set LF to be paragraph, I’d stick to your solution, because it’s really most elegant - replacing double breaks with something unique, then replacing rest with spaces, then replacing uniques back to paragraphs. But opening the file so that LFs are line breaks would make my solution better one (more straightforward), and only requires choosing CRLFs for paragraphs in formatted text filter.

You just don’t want to listen.

I still don’t know what actual format your real file is. If you are just clueless about formats, or deliberately post wrong-named files to confuse those who try to help.

However, you don’t take time to stop and think. Even if you just have an ODF, and it already has its paragraph breaks, you might use a preliminary step to convert $ to line break (see here) and follow.

“Non-printing paragraph mark” creates paragraph break. So, if you want to get rid of this mark, you effectively remove new paragraph break in this place. I suppose it’s what you actually want to do; e.g., this might happen if you import some “formatted” plain text, or a scan result, and you assume that if there’s not a dot before the paragraph break, then this paragraph break should be eliminated.

LO’s own Find and Replace dialog is unable to remove paragraph breaks in Regex mode (it operates in one paragraph’s boundaries only; the only explicit exception is removing empty paragraphs by using ^$). So, you might consider AltSearch extension for that.

The proper find string for that task would be ([a-zA-Z0-9]+)$ (note the parentheses), and replace would be \1 (first back-reference plus a space).

EDIT (2017-05-17): the file that you finally attached is a bright demonstration why the samples must be provided to make it clear what’s the problem is.

The file has only one paragraph in it, and multiple line breaks (not paragraph breaks!). Each “wanna-be-paragraph” is delimited by two successive line breaks; all other line breaks should be converted to spaces. So, taking into account LibreOffice’s List of Regular Expressions (saying: \n in the Find text box stands for a line break that was inserted with the Shift+Enter key combination; \n in the Replace text box stands for a paragraph break that can be entered with the Enter or Return key), we need just these two steps with usual Find and Replace dialog and Regular Expressions checked:

  1. Search for \n\n and replace with \n.
  2. Search for \n and replace with (single space).

That’s it. If you would provide the sample right at the start (by e.g. sharing in a public service like DropBox), the solution would be really quick.

EDIT 2: the attached file, despite being named Bertrand.odt, actually is a plain text file that has LFs as line breaks. On opening with LO, the breaks are treated depending on LO’s formatted text settings, where you may choose if LFs, CRLFs or CRs are treated as paragraph breaks. So, opening it so that LFs stay as line breaks, allows for my solution above.

EDIT 3: here is the exact sequence to do with the file Toughening up.odt (that is actually another plain text file, again):

  1. Rename it to Toughening up.txt
  2. Open LibreOffice Start Center, File-Open, and choose Text - Choose Encoding (*.txt) in File Type drop-down list. Select and open the Toughening up.txt. In ASCII Filter Options dialog, choose CR & LF as Paragraph break
  3. Search for ([.])\n and replace with $1\n.
    • Check if all generated paragraph breaks are correct.
    • You might want to extend the repertoire of characters in square brackets.
    • Another (equivalent) find-replace pair is (?<=[.])\n\n - using Look-behind assertion.
  4. Search for \n and replace with (single space).

You mean, use the ([a-zA-Z0-9]+)$ (note the parentheses), and replace code \1 (first back-reference plus a space) after I have installed AltSearch Extn?

Thanks in advance!

Yes, these are for AltSearch. For usual Search and Replace, the backreference would be $1, not \1 (but it will not fork in this case, as I mentioned).

Sorry, there is a problem. My ver is Version 4.0.3.3 (Build ID: 400m0(Build:3)) within Knoppix 7.2.0 and I am not in a position to upgrade the version or Knoppix (old laptop).
The extension is problematic, it loops endlessly, it seems, when I have ticked regex and clicked Find!

It might be that it is just takes long time to finish (on a big document). Try to test it on a new document with a few lines to make sure if it works or not.

If it doesn’t, then you could make a multi-step replacement. E.g., you could try to replace all $s with e.g.
{NEWLINE}, then search for those \{NEWLINE\}s that are after your pattern and replace them with spaces, and then replace \{NEWLINE\}s left with \n.

Okay, I will try, and then get back.

I forgot to add that if you’d try to remove all paragraph breaks, then you must remember that there’s a limit for one paragraph length (for that old version, it’s 64K characters IIRC). So, for a big document, it might not work.

The extension is showing this error:
FindNextInBlock: Error 9: Index out of defined range. (line: 2561)
So do you wish me to try one paragraph at a time?
I would not like to make it complicated with double replacements. It is logically difficult to visualise.

I tried with ([a-zA-Z0-9]+)$ for a selection of text having the pattern multiple no, of times.
I had the result:

Next occurrence of searched expression  "([a-zA-Z0-9]+)$"  not found. 
 
 Searching inside of selection has been finished.

Well, I’m out of ideas then.

Thank you for your benevolent gesture of trying to help me.

Is there no simple macros? Not elaborate codes?

So you do not mean the sign $ but the sign ?

If your task is, to join single line paragraphs, then you can do it with “AutoCorrect”.

First set the option: menu Tools > AutoCorrect Options, then tab Options. Find the entry Combine single line paragraphs if length greater than 50%. Check its checkbox in column [M]. Double-click on the term 50%, then you can edit it. A value of 1% should work for you. OK. OK.

Now select the lines you want to join. Then menu Formats (in case of LO4) > AutoCorrect > Apply.

Thanks Regina! I completed the work manually by find option which worked. But I will remember to check and report.

My solution assumes, that there are paragraph breaks, but your finally provided text does not contain paragraph breaks but line breaks. So my solution will not work for your text. Your task was much clearer, if you would have provided an example file from the beginning.

The placeholder for ‘EndOfParagraph’ ($) placed as the only character in '‘Search For:’ and the space in ‘Replace With:’, RegEx enabled, will do the first step. This will not only work for empty paragraphs or single line paragraphs. If there are additional replacements wanted a second step is necessary.

You may also check if the extension ‘AltSearch’ (Aölternative Serach & Replace / Installation from .oxt needed) has more capabilities. It is not very efficient with th kind of task.