Looking Inside The Regex Engine

As was mentioned earlier: the order of the characters inside a character class does not matter. gr[ae]y matchesgrey in Is his hair grey or gray?, because that is the leftmost match. We already saw how the engine applies a regex consisting only of literal characters. Now we'll see how it applies a regex that has more than one permutation. That is: gr[ae]y can match both gray and grey.
Nothing noteworthy happens for the first twelve characters in the string. The engine fails to match g at every step, and continues with the next character in the string. When the engine arrives at the 13th character, g is matched. The engine then tries to match the remainder of the regex with the text. The next token in the regex is the literal r, which matches the next character in the text. So the third token, [ae] is attempted at the next character in the text (e). The character class gives the engine two options: match a or match e. It first attempts to match a, and fails.
But because we are using a regex-directed engine, it must continue trying to match all the other permutations of the regex pattern before deciding that the regex cannot be matched with the text starting at character 13. So it continues with the other option, and finds that e matches e. The last regex token is y, which can be matched with the following character as well. The engine has found a complete match with the text starting at character 13. It returns grey as the match result, and looks no further. Again, the leftmost match is returned, even though we put the a first in the character class, and gray could have been matched in the string. But the engine simply did not get that far, because another equally valid match was found to the left of it. gray is only matched if you tell the regex engine to continue looking for a second match in the remainder of the subject string after the first match.

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