<?php
error_reporting(-1);
$words = ['штаны', 'ёлка', 'слово'];
$regexps = ['~^[а-я]+$~u', '~^[а-яё]+$~u'];

foreach($words as $word)
	foreach($regexps as $regexp)
		printf(
			"Regexp: '%s', word: '%s', match: %s\n",
			$regexp,
			$word,
			preg_match($regexp, $word) ? 'true' : 'false'
		);