Preg difference between /^a|b|c$/ and /^a$|^b$|^c$/

Question

To me they look the same... why are they different in practice?

echo preg_match("/^a|b|c$/", "aaaa");

echos 1

and echo preg_match( "/^a$|^b$|^c$/", "aaaa");

echos 0

Answer 1

/^a|b|c$/ matches all values starting with "a", containing "b" or ending with "c".
/^a$|^b$|^c$/ matches just "a" or "b" or "c" and no other values.

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

demo on ideone.com

Answer 2

In order for the first one to be exactly the same as the second one you must use a group:

echo preg_match("/^(a|b|c)$/", "aaaa");

Regex example

Otherwise it means it has to match ^a or b or c$