Bug introduced in 11.3 or earlier, persisting through 12.0 or later and fixed in 12.3.1 or earlier

[CASE:4352435] and [CASE:4354378]

In Mathematica 11.3 and 12.0,


incorrectly returns


In earlier versions (10.1, 10.3, 11.0, 11.1) executing the above code causes the kernel to hang, requiring it to be quit, so this appears never to have worked correctly. See also the Answer below.

In addition, in the earlier versions, the following also hangs:

ReplaceList[{2, 3}, {u_, Except[1 | u_] ..} :> True]

Using Replace instead of ReplaceList is fine; SequenceCases behaves incorrectly like ReplaceList.

The issue is also seen with orderless matches (which presumably are represented by alternatives internally); for example,

 ReplaceList[{2 x, 2 x}, {2 u_, Except[2 u_]} :> True]

incorrectly returns


in 11.3 and 12.0.

-- The above has been revised as a result of feedback and further testing.

  • $\begingroup$ ReplaceList and SequenceCases work for me in version 12. $\endgroup$
    – C. E.
    Nov 9, 2019 at 11:36
  • $\begingroup$ I suggest you check these examples with a new kernel. Also report your operating system, in case this is significant. $\endgroup$
    – mikado
    Nov 9, 2019 at 11:41
  • $\begingroup$ @C.E. -- Thanks for checking; sounds like I should upgrade. $\endgroup$ Nov 9, 2019 at 12:06
  • $\begingroup$ Kernel hang confirmed in v10.1.0 under Windows x64. $\endgroup$
    – Mr.Wizard
    Nov 9, 2019 at 13:37
  • $\begingroup$ ReplaceList[{2, 3}, {u_, Except[1 | u_] ..} :> True] evaluated instantly and returned {True} in V11.3.0 running on MacOS 10.13.4 $\endgroup$
    – m_goldberg
    Nov 9, 2019 at 14:19

1 Answer 1


Update. The bug is confirmed by the tech support: [CASE:4352435].


ReplaceList[{2, 3}, {u_, Except[1 | u_] ..} :> True]

Mathematica 8.0.4 prints error:

Except::named: Named pattern variables are not allowed in the first argument of Except[1|u_].

and returns {}, but versions 11.3 and 12.0 both return {True} without any messages.

What seems to be a real bug in the latest versions is that each of the following two lines also returns {True}:

ReplaceList[{2, 2}, {u_, Except[u_ | 1]} :> True]    
SequenceCases[{2, 2}, {u_, Except[u_ | 1]} :> True]

while the pattern doen't match:

MatchQ[{2, 2}, {u_, Except[u_ | 1]}]

Replace works as expected though:

Replace[{2, 2}, {u_, Except[u_ | 1]} :> True]
Replace[{2, 3}, {u_, Except[u_ | 1]} :> True]
{2, 2}


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