Description:
------------
Modern Russian alphabet contains 33 letters.
Standard UTF8 rage covers 32 of most common, but misses one ('ё').

Standard
U+0410	А
…
U+044F  я

Exceptions:
U+0401	Ё
U+0451	ё
http://www.utf8-chartable.de/unicode-utf8-table.pl?start=1024

[а-я] range should include 'ё' (and [А-Я] — 'Ё') letter, but actually do not.

Test script:
---------------
$ cat cyr_io.php
<?php
$str = "еще";
$valid_string_expr = '/(*UTF8)[^а-я]/';
if (filter_var($str, FILTER_VALIDATE_REGEXP, array("options"=>array("regexp"=>"$valid_string_expr"))) === FALSE)
  echo ("string contains only cyrillic small letters\n");
else
  echo ("string contains NOT only cyrillic small letters\n");

$str = "ещё";
if (filter_var($str, FILTER_VALIDATE_REGEXP, array("options"=>array("regexp"=>"$valid_string_expr"))) === FALSE)
  echo ("string contains only cyrillic small letters\n");
else
  echo ("string contains NOT only cyrillic small letters\n");
?>

Expected result:
----------------
$ php cyr_io.php
string contains only cyrillic small letters
string contains only cyrillic small letters

Actual result:
--------------
$ php cyr_io.php
string contains only cyrillic small letters
string contains NOT only cyrillic small letters