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    XY|YX

    XYZ|XZY|YXZ|YZX|ZXY|ZYX

    (?:X|Y){2}    (?# for the first example)
    (?:X|Y|Z){3}  (?# for the second)

(?:(?(1)NOTMATCH|(X))|(?(2)NOTMATCH|(Y))|(?(3)NOTMATCH|(Z))){3}