Note that preg_replace() is often faster than ereg_replace().
This one should work, too:
$clean_text = preg_replace ("/([^a-z]|\s)/i", '', $input_text);
note that the small "i" at the end of the pattern indicates that you want the pattern to be case-insensitive.
Also \s means any spaces, including white space, tabs, and new lines. If you want to remove white space only, replace \s with a single space.