Allowing whitespace when validating form entries

[roondog]

I've got a form i'm testing at www.roondogid.co.uk/login/register.php

i've got it all working except it won't let me have any gaps in the gamertag
section.
the code for that section is:

if (eregi ('^[[:alnum:]]{2,15}$', stripslashes(tr im($_POST['gamertag'])))) {
$g = escape_data($_P OST['gamertag']);
} else {
$g = FALSE;
echo '<p class="error">P lease enter your gamertag!</p>';
}

all help appreciated.