$path='Hello\ World There is\ a\ dog';
$split=preg_split('/(?<!\\)\s+/',$path);
print_r($split);

returns:

Warning: Compilation failed: lookbehind assertion is not fixed length at offset 9 in /home/paj/projects/pear/core/test.php on line 17
/home/paj/projects/pear/core/test.php(17) : Warning - Compilation failed: lookbehind assertion is not fixed length at offset 9

$path='Hello\ World There is\ a\ dog';
$split=preg_split('/(?<!\\\)\s+/',$path);
print_r($split);

does not, but does not work as expected with others cases.

It seems to work with 4.3.0, a test url:
http://devel.akbkhome.com/regex.php
which runs 4.3.0, the 1st expression works correclty

pierre