How to search and replace for special characters in LibreOffice / OpenOffice Writer?

You can certainly use regular expressions in LibreOffice 3.4 as I have just tried it:

There's a fuller list of what can and can't be used here:

http://help.libreoffice.org/Common/List_of_Regular_Expressions

Edit: There's some comments on how to deal with paragraph marks here:

http://www.oooninja.com/2007/12/example-regular-expressions-for-writer.html

Even better (in my opinion) to do it with AltSearch. See this post. In the GUI of AltSearch You'll easily find codes all the non-printing chars and much more.