Are there example Strings for unittests? (e.g. with control characters)

I have to write several unittests in a software of ours. In many ocasions I came across Strings which are Internally set/User set or even persisted (e.g. Database/XML etc.)

Does anyone know of a character-set or string(s) to do simple tests, whether input is equal to output?

I know how to test, my question regards what to test.

For numbers you e.g test values around 0 or - + numbers or if they're retrieved from a string, floating point numbers and integer numbers.

But for Text/String I'm not aware of standard sequences to test.