There is some exported spreadsheet with numbers, preceding with space. Some
spaces also divide thousands, even it's not written in their format.
The combination of space and digit can by find via
([:space:][:digit:])
But trying to replace the combination with the same digit fails. The [:digit:]
in replacement string is considered as the text "[:digit:]". The & is considered
as the whole search criterion. Trying to mark a part of search criterion with
()\1 failed.

Is it possible to make a replacement with part of the search criterion? If no,
it's a shortage of OO regular expressions (enhancement). If yes, it's not
covered in the help (bug).