php regular expression to verify username

/** 
 * 检查用户名是否符合规定 
 * 
 * @param STRING $username 要检查的用户名 
 * @return TRUE or FALSE 
 */ 
function is_username($username) 
{ 
$strlen = strlen($username); 
if (!preg_match("/^[a-zA-Z0-9_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]+$/", 
$username)) //开源软件:phpfensi.com 
{ 
return false; 
} elseif (20 < $strlen || $strlen < 2) 
{ 
return false; 
} 
return true; 
}

function isPWD($value,$minLen=5,$maxLen=16){ 
$match='/^[\\~!@#$%^&*()-_=+|{}\[\],.?\/:;\'\"\d\w]{'.$minLen.','.$maxLen.'}$/'; 
$v = trim($value); 
if(emptyempty($v)) 
return false; 
return preg_match($match,$v); 
}

function isEmail($value,$match='/^[\w\d]+[\wd-.]*@[w\d-.]+\.[\w\d]{2,10}$/i')

{ 
$v = trim($value); 
if(emptyempty($v)) 
return false; 
return preg_match($match,$v); 
}