Description:
------------
In line with the processing of the Cyrillic alphabet in the regular expression key /i (case-insensitive search) error occurs

Windows Server 2012 R2
IIS 8.5
PHP 7.0.9 x64

Test script:
---------------
<?php
	header('Content-Type: text/plain; charset=windows-1251');

	$RegularExpression = '/^[a-zа-я]+$/i';

	$TestStrArray = array('ABC', 'abc', 'АБВ', 'абв');

	foreach ($TestStrArray as $TestStr)
	{
		$Result = preg_match($RegularExpression, $TestStr);

		echo $TestStr;

		if ($Result === false)
		{
			echo ' - Error'."\r\n";
		}
		else
		{
			echo ' - '.($Result === 0 ? 'not found' : 'found')."\r\n";
		}
	} // foreach
?>

Expected result:
----------------
ABC - found
abc - found
АБВ - not found
абв - found

Actual result:
--------------
ABC - found
abc - found
АБВ - found
абв - found