Regexp usage example (including answer by Vaclav Pech)

Is there a sample of how to use regexp to validate input?
E.g. to check that a string does not start with a digit and does not contain spaces

Answer (by Vaclav Pech):
FloatingPointConstant is the example that you are looking for. Just open it in MPS (Control/Cmd + N) and check the definition of the _FPNumber_String datatype.