validating username

i have used the following code to validate the username it is working fine ============================================= if( $username == "" || !preg_match("/^[a-z0-9]+(?:_[a-z0-9]+)?$/i", $username) ) { $error.="User name cannot be blank or has special characters"; } ============================================= it does not accept UNDERSCORE at the beginning or end however while i was testing with different special characters except for # the validation works fine for all other special characters. for example if i enter the user name as = abc#123 in this case # sign and what comes after # sign is being ignored. so in this case the username is being read as abc ONLY and not abc#123 this is very strange, how can i still validate # sign and tell the user that # sign is not a valid username like i have been doing with any other special characters like = !@$........... please advice. thanks.