Match two spaces with incremental search

Question

When searching using:

C-s SPC SPC

Also matches single spaces. I want to match exactly two space.

Answer 1

Use M-s SPC during Isearch to toggle matching whitespace literally. When matching literally, each SPC char you type is matched individually. (This used to be the default Emacs behavior, BTW.)

To configure this as the default behavior customize option search-whitespace-regexp to nil. (M-x customize-option search-whitespace-regexp.)

See the GNU Emacs manual, node Special Isearch.

C-h v search-whitespace-regexp tells us this (showing the default value):

search-whitespace-regexp is a variable defined in isearch.el.

Its value is "\\s-+"

Documentation:

If non-nil, regular expression to match a sequence of whitespace chars.

When you enter a space or spaces in the incremental search, it will match any sequence matched by this regexp. As an exception, spaces are treated normally in regexp incremental search if they occur in a regexp construct like [...] or *, + or ?.

If the value is a string, it applies to both ordinary and regexp incremental search. If the value is nil, or isearch-lax-whitespace is nil for ordinary incremental search, or isearch-regexp-lax-whitespace is nil for regexp incremental search, then each space you type matches literally, against one space.

You might want to use something like "[ \t\r\n]+" instead. In the Customization buffer, that is [ followed by a space, a tab, a carriage return (control-M), a newline, and ]+.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.3 of Emacs.

Answer 2

Using regexp incremental search solves the problem, if you escape the space characters:

C-M-s \SPC\SPC

You can also use search and replace without the backslashes, if that's what you're after:

M-% SPC SPC

Answer 3

Using regexp search helps, but you can also make sure of literal spaces for the search using C-q SPC to quote a space into the search.

To riff off of @Rovanion's answer then, using isearch-forward-regexp:

C-M-s C-q SPC C-q SPC

That will search for two consecutive spaces. Interestingly, isearch-forward-regexp requires discrete matches. Searching aaa for aa will only match once and not again at the second character.