Why does Shortest not work inside SequencePosition?

Asked Jun 05 '19 at 17:56

Active Jun 13 '19 at 07:53

Viewed 103 times

Why does Shortest not work in a SequencePosition call? The following two commands return the same answer:

SequencePosition[{1, 2, 3, 4, 5}, {a_, ___, b_}]
(*    {{1, 5}, {2, 5}, {3, 5}, {4, 5}}    *)
SequencePosition[{1, 2, 3, 4, 5}, {a_, Shortest[___], b_}]
(*    {{1, 5}, {2, 5}, {3, 5}, {4, 5}}    *)

I would prefer it if the second one returned

(*    {{1, 2}, {2, 3}, {3, 4}, {4, 5}}    *)

Do you know how to achieve this behavior?

Notice that the closely related StringPosition works as expected:

StringPosition["12345", a_ ~~ ___ ~~ b_]
(*    {{1, 5}, {2, 5}, {3, 5}, {4, 5}}    *)
StringPosition["12345", a_ ~~ Shortest[___] ~~ b_]
(*    {{1, 2}, {2, 3}, {3, 4}, {4, 5}}    *)

edited Jun 13 '19 at 07:53

asked Jun 05 '19 at 17:56

Roman

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Forwarded to Wolfram Support as unexpected behavior. – Roman Jun 09 '19 at 11:43
Confirmed by Wolfram Support as unexpected behavior [CASE:4269719]. – Roman Jun 13 '19 at 07:53
It's no surprise StringPosition doesn't share the problem - it is based on a common (textual) regular expression matcher (was it PCRE?), while the pattern matcher in Mathematica is their own. – kirma Jun 13 '19 at 10:54

Why does Shortest not work inside SequencePosition?

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