PHP: Check if my string only contains numbers and "X" character

I retrieve the nexts values of my size products: 30X50 or 20x20

I want add the "cm" measure if this values contais the character x or X and only numbers.

@if($myvalue contains only the character x and numbers)
  cm // like 20x20cm
@endif

I need a regular expression. But I do not know how to do it

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