Think as follows.

• One possibility is that there is no y. Then [a-xz]* matches it, with [a-wyz]* matching an empty string.

• Another possibility is that there is no x. Then [a-wyz]* matches the whole string, with [a-xz]* matching an empty string.

• Finally, the string might contain both an x and a y. Then [a-xz]* matches the part up to the first y and [a-wyz]* matches the rest.

• Note that it is not possible to match a string that has an x after a y. The only way to match a y is to use [a-wyz]*. But then you can never get another x.