search-whitespace-regexp

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.