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I am trying to match a list of strings with a pattern, in which there are several possibilities for the characters to be matched. To me, the following looks the most natural way:

 StringMatchQ[{"12", "23", "34", "45"}, ___ ~~ Alternatives @@ {"1", "4"} ~~ ___]

what returns as expected:

{True, False, True, True}

However, I've found out the following does work too (and it's faster when the list is very large):

StringMatchQ[{"12", "23", "34", "45"}, ___ ~~ {"1", "4"} ~~ ___]

{True, False, True, True}

Why does this work? Why are we able to specify the different possibilities with a list instead of with Alternatives?.

I have also found out we do not even need to evaluate Alternatives properly, the following does work as well:

 StringMatchQ[{"12", "23", "34", "45"}, ___ ~~ Alternatives @ {"1", "4"} ~~ ___]

{True, False, True, True}

even though the head of List is not replaced:

Alternatives@{"1", "4"}

what yields the (undefined?) symbolic expression:

Alternatives[{"1", "4"}]

Why these last two ways work and where is it documented?

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Look at the FullForm of the pattern; you will notice that it becomes a StringExpression:

FullForm[___ ~~ {"1", "4"} ~~ ___]

(* Out: StringExpression[BlankNullSequence[], List["1", "4"], BlankNullSequence[]] *)

The second bullet point in the "Details" section of the documentation of StringExpression lists the types of objects that can appear in StringExpression, among which is

"{$p_1$, $p_2$, ...} or $p_1$ | $p_2$ |... -> a pattern matching at least one of the $p_i$".

For your second question, I interpret Alternatives[{"1", "4"}] to mean "a pattern object that represents any of the patterns {"1", "4"}", which I would then interpret as the pattern {"1", "4"} itself. Then you go back to what we discussed above.

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