I think (and hope) choroba has hit your definition of "special characters" on the head.... even if it is a specification that is utterly non-standard, arcane, and imprecise.

You should have provided that specification because "special characters" has special meanings in certain contexts -- and because the solution offered keeps some numbers and deletes others... in conformity with your example. But defining a spec based on a small example/sample is a risky and often ambiguous process.