Trouble in understanding the behavior of search-backward-regexp

[Alain Cochard]

Hello.  The trouble concerns the regular expression 

   [[:blank:]]+ 

for which I read that it matches any sequence of spaces and tabs.

Let me start with search-forward-regexp, with which I have no problem.
Let us say I have the following sentence:

   foo                                     bar

with spaces and tabs between the two words

If I put the cursor at the beginning and I use 'search-forward-regexp'
with this [[:blank:]]+, I end up with the cursor right on the 'b',
which is what I expect.

Now, if I put the cursor at the end of that sentence and use
search-backward-regexp, again with [[:blank:]]+, I would expect to end
up on the second 'o' of 'foo'.  Instead, I end up somewhere between
the two words, which is very mysterious to me (I have tried with
several combinations of emacs-version/distributions/hardware).

Thanks in advance for any tip.
Alain