Or you could just use the much faster str_replace() on the whole log:
$log = //put your file here in any way you please
$search = array('curse', 'curses', 'Curse', 'Curses');
$replace = array('<strong>curse</strong>',
'<strong>curses</strong>',
'<strong>Curse</strong>',
'<strong>Curses</strong>');
$newLog = str_replace($search, $replace, $log);