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Bug introduced in 10.1 and fixed in 10.2.0

In v10.1.0.0 on Linux, when Blank[] is used in Alternatives with pattern that can match a sequence, whole pattern unexpectedly matches sequences of any length.

ClearAll[matchLists]
matchLists[patt_] :=
    MatchQ[#, patt] & /@ {{}, {1}, {1, 2}, {1, 2, 3}, {1, 2, 3, 4}}

matchLists[{_ | PatternSequence[]}]
(*{True, True, True, True, True}*)
matchLists[{_ | PatternSequence[_, _, _]}]
(*{True, True, True, True, True}*)
matchLists[{_ | __}]
(*{True, True, True, True, True}*)
matchLists[{_ | Repeated[_, {2, 3}]}]
(*{True, True, True, True, True}*)

If Blank is non-empty, or is inside Pattern, everything works as expected:

matchLists[{x_ | PatternSequence[]}]
(*{True, True, False, False, False}*)
matchLists[{_Integer | PatternSequence[_, _, _]}]
(*{False, True, False, True, False}*)
matchLists[{y_ | __}]
(*{False, True, True, True, True}*)
matchLists[{_Integer | Repeated[_, {2, 3}]}]
(*{False, True, True, True, False}*)

In all previous versions I have available: 8.0.0, 9.0.1 and 10.0.2, all above patterns work as expected:

matchLists[{_ | PatternSequence[]}]
(*{True, True, False, False, False}*)
matchLists[{_ | PatternSequence[_, _, _]}]
(*{False, True, False, True, False}*)
matchLists[{_ | __}]
(*{False, True, True, True, True}*)
matchLists[{_ | Repeated[_, {2, 3}]}]
(*{False, True, True, True, False}*)

matchLists[{x_ | PatternSequence[]}]
(*{True, True, False, False, False}*)
matchLists[{_Integer | PatternSequence[_, _, _]}]
(*{False, True, False, True, False}*)
matchLists[{y_ | __}]
(*{False, True, True, True, True}*)
matchLists[{_Integer | Repeated[_, {2, 3}]}]
(*{False, True, True, True, False}*)

Can someone confirm this behavior?

Reported to WRI as [CASE:3059582]

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5
  • 3
    $\begingroup$ Confirmed on 10.0 (linux, mma online), 10.1(windows 8.1 ,with string pattern fix), 10.1(OS X 10.10.3), 9.0.1 (windows xp), 8.0.4 (windows xp), $\endgroup$
    – vapor
    Apr 16, 2015 at 17:14
  • 1
    $\begingroup$ @Felix thanks. To be clear, you confirm buggy behavior in 10.1 and expected behavior in other versions? $\endgroup$
    – jkuczm
    Apr 16, 2015 at 17:27
  • 1
    $\begingroup$ Yes, I got exactly the same results as you did. $\endgroup$
    – vapor
    Apr 16, 2015 at 17:30
  • $\begingroup$ @Felix OK, thanks again. $\endgroup$
    – jkuczm
    Apr 16, 2015 at 17:31
  • 3
    $\begingroup$ This has been fixed in our latest development builds. Thanks for reporting it!! $\endgroup$
    – Stefan R
    May 7, 2015 at 16:46

1 Answer 1

Reset to default
8
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This bug has been fixed in version 10.2.

$Version
(* "10.2.0 for Linux x86 (64-bit) (July 6, 2015)" *)

matchLists[patt_] := 
    MatchQ[#, patt] & /@ {{}, {1}, {1, 2}, {1, 2, 3}, {1, 2, 3, 4}}

matchLists[{_ | PatternSequence[]}]
matchLists[{x_ | PatternSequence[]}]
(* {True, True, False, False, False} *)
(* {True, True, False, False, False} *)

matchLists[{_ | PatternSequence[_, _, _]}]
matchLists[{_Integer | PatternSequence[_, _, _]}]
(* {False, True, False, True, False} *)
(* {False, True, False, True, False} *)

matchLists[{_ | __}]
matchLists[{y_ | __}]
(* {False, True, True, True, True} *)
(* {False, True, True, True, True} *)

matchLists[{_ | Repeated[_, {2, 3}]}]
matchLists[{_Integer | Repeated[_, {2, 3}]}]
(* {False, True, True, True, False} *)
(* {False, True, True, True, False} *)
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