regexp trhouble

ive some kind of basic trhouble, ive assemble a very long regexp and this is the only part missing to finish it.
how can i make a regexp that matchs a text with a lot of spaces and in the end something that dont be the text "</b>".
here is an example:

header('content-type: text/plain;');
$texto = <<< el_texto
          </b>
el_texto;
;

$regexp = '((\s(?=\s*(?!</b>))))';

preg_match_all($regexp,$texto,$matches);
var_dump($matches);

Code (markup):

that code actually return results even when i dont need it, i need to a regular expresion that return te spaces only when is not followed by </b>, or dont return nothing if its pressent.

 

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