Regular expression to match two numbers that are not equal

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My question was:

I was wondering how can I match two digits but which are not the same. So, it would be fine to match 12, but not 11.

What I have till now is: I have to match strings like “P12”, and I’ve done it with this regex:

^P([1-6]{1})([1-6]{1})$

But now my problem is how to match only strings like P12 or P32 where the numbers do not repeat.

Any help or guidance to reading materials will be grateful.

edit:  this is what worked:

^P([1-6]{1})(?!\1)([1-6]{1})$

 The answer, by Ignacio Vazquez-Abrams, was:

Use this:

^P((1[2-6])|(2[13-6])|(3[124-6])|(4[1-356])|(5[1-46])|(6[1-5]))$