Open any text file and try searching for pipes (|). This character is common when editing Wikimedia pages or templates externally.
IF by accident you've forgotten to move back to the normal plain text mode and the search field contains "||" or starts by a "|", then this regexp will mean searching for EMPTY strings.
The search & replace will then run in an infinite loop and you'll have NO other option than killing Notepad++, loosing your last unsaved edits.
The regexp may not be used to find EMPTY text, or if it does then for each occurence if must NOT start at the current position in the text, but MOST only return an empty selection starting at the NEXT character. Otherwise it will infinitely return the same EMPTY selection at the current edit position. And if you search and replace, it will infinitely try to append the replacement text.
The simplest fix should NOT return empty selections (even if this is what the regexp means, including searching for "$" in a regexp, which will infinitely match an EMPTY text at end of the current line)