This is a recent problem I had brought to my attention on my YaBBSE board and having tested it here we have exactly the same problem with SMF.
Carry out a search for a word when entering a username in the "by user" box and select "search for topics posted by someone"
If the username has a space in it (eg James Woodcock) , you will get a zero result even though a search without quoting a username brings up the right result.
I had to amend my YaBBSE search.php on a single line to get it working at home
$userspec = trim($userspec);
$userspec = preg_replace("/[^0-9A-Za-z#%+,-\.@^_]/", "", $userspec);
modify to
$userspec = trim($userspec);
$userspec = preg_replace("/ [^0-9A-Za-z#%+,-\.@^_]/", "", $userspec);
Spot the "space" added after the slash in the preg_replace.
I haven't found any problems in running search with the space added.
I think I just fixed this in the cvs, although I'm not sure... we'll see when I upload it.
-[Unknown]