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  • $\begingroup$ I'm on the road, but my first guess is that when q matches X, the replacement results in the same expression. Would that cause it to stop? $\endgroup$ Commented Oct 14, 2024 at 1:57
  • $\begingroup$ @MichaelE2 I see now. Got it thanks! Should I post a solution myself? $\endgroup$ Commented Oct 14, 2024 at 2:04
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    $\begingroup$ @MichaelE2 I found a workaround, but cannot find no satisfactory answer to my question itself... $\endgroup$ Commented Oct 14, 2024 at 2:36
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    $\begingroup$ Does this help with finding an explanation: {a, X, c, d, X, b, X} //. {{p___, X, q_, r___} :> (Print[{"q" -> q, #}]; #) &@{p, q, X, r}}? It prints the replacement for each match. lericr has already explained the point in my comment. $\endgroup$ Commented Oct 14, 2024 at 12:52
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    $\begingroup$ If you make your rule {{p___, X, q : Except[X], r___} :> {p, q, X, r}} or {{p___, X, r__} :> {p, r, X}}, it will work. $\endgroup$ Commented Oct 14, 2024 at 15:13