Named string patterns in Alternatives, why does it depend on order?

When using the same name for part of patterns inside Alternatives how come it behaves differently for strings than in normal patterns?

{StringMatchQ["ab", (x_ ~~ "c") | ("a" ~~ x_)],
 StringMatchQ["ab", ("a" ~~ x_) | (x_ ~~ "c")]}
(* False, True *)

{MatchQ[{1, 2}, {x_, 3} | {1, x_}],
 MatchQ[{1, 2}, {1, x_} | {x_, 3}]}
(* True, True *)


StringMatchQ["ab", (x_ ~~ "c") | (x_ ~~ "b")]
(* False *)

Answer

As @OleksandrR pointed out string patterns are translated to PCRE

The tutorial,in the Implementation Details section, mentions PatternConvert that shows the translation used:

StringPattern`PatternConvert[(x_ ~~ "c") | ("a" ~~ x_)]
(* {"(?ms)(?:(.)c|a(?:\\1))", {{Hold[x], 1}}, {}, Hold[None]} *)

The relevant part here is (?:(.)c|a(?:\1)) when I was expecting something like: (?:(?.)c|a(?.))

Comments

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