<?php
echo preg_match('/\b\pL{2,4}\b/ui', 'очень длинные слова');
echo preg_match('/\b\pL{2,4}\b/ui', 'тут короткие слова');
echo preg_replace('/\w{2,4}\b/ui', '<$0>', 'оче-нь тут слова и так дальше');
Warning: preg_match(): Compilation failed: PCRE does not support \L, \l, \N, \P, \p, \U, \u, or \X at offset 3 in /in/u3CiJ on line 2
Warning: preg_match(): Compilation failed: PCRE does not support \L, \l, \N, \P, \p, \U, \u, or \X at offset 3 in /in/u3CiJ on line 3
оче-нь тут слова и так дальше
Output for 4.3.0 - 4.3.10
Warning: Compilation failed: PCRE does not support \L, \l, \N, \P, \p, \U, \u, or \X at offset 3 in /in/u3CiJ on line 2
Warning: Compilation failed: PCRE does not support \L, \l, \N, \P, \p, \U, \u, or \X at offset 3 in /in/u3CiJ on line 3
оче-нь тут слова и так дальше