<?php
echo preg_match("/^a|b|c$/", "aaaa"); // 1
echo preg_match("/^a|b|c$/", "zbaa"); // 1
echo preg_match("/^a|b|c$/", "aaac"); // 1
 
echo preg_match("/^a$|^b$|^c$/", "aaaa"); // 0
echo preg_match("/^a$|^b$|^c$/", "zbaa"); // 0
echo preg_match("/^a$|^b$|^c$/", "aaac"); // 0
 
echo preg_match("/^a$|^b$|^c$/", "a"); // 1
echo preg_match("/^a$|^b$|^c$/", "b"); // 1
echo preg_match("/^a$|^b$|^c$/", "c"); // 1
echo preg_match("/^a$|^b$|^c$/", "d"); // 0

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