XY|YX

##</code><code>##

    XYZ|XZY|YXZ|YZX|ZXY|ZYX

##</code><code>##

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

##</code><code>##

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