#([а-я]+)#i

$str = 'Привед';

#[а-яё]#i

#[а-яё]#iu

if (preg_match("#[а-яё]#i\s\-", $str))

if (preg_match("#[а-яё\-\s]#i", $str))

if (preg_match("#^[а-яё\-\s]+$#i", $str))

<?php
$lastname=$_POST["data"];
function check_rus($lastname) 
{
    setlocale(LC_CTYPE, "ru_RU.UTF-8");
    if (preg_match("#^[а-яё\-\s]+$#i", $lastname))
	{
        return true;
    }
    return false;
}


$lastname=ucfirst(strtolower(trim($lastname)));
if(check_rus($lastname))
{
echo "рус";
}
else 
{
echo "eng";
}
?>

setlocale(LC_CTYPE, "ru_RU.CP1251");

function check_rus($str) 
{
    if (preg_match("/^([а-яёЁ\s-]+)$/iu", $str)) 
	{
        return true;
    }
    return false;
}
$lastname = $_POST['data'];
$lastname = ucfirst(strtolower(trim($lastname)));
var_dump(check_rus($lastname));

function check_rus($str) 
{
    return (boolean)preg_match("/^([а-яёЁ\s-]+)$/iu", $str);
}
$lastname = 'русский букв Ё';

var_dump(check_rus($lastname));

<form action="rus_eng.php" method=POST>
<input type=data name=searchword size=30></input>
<input type=submit value="Искать"></input><br><br>
</form>

<form action="rus_eng.php" method=POST>
    <input type="text" name="searchword" size=30></input>
    <input type="submit" value="Искать"></input><br><br>
</form>

<?php
function check_rus($str) 
{
    $str = iconv('CP1251', 'UTF-8', $str);
    if (preg_match("/^([а-яёЁ\s-]+)$/iu", $str)) 
        return true;

    return false;
}

$lastname = $_POST['searchword'];
var_dump(check_rus($lastname));
?>

<input type="text" name="searchword" size="30">

<input type="text" name="searchword" size="30" />