"Numeric" means numeric, such as 1.23 or -1e-16, while pi/2 has to be interpreted. The CellEditcallback (you can use this term to let Google find more details for you) can call eval() to evaulate this expression. But then a small test would be fine to control that no mad user tries to input '!format C:', because even this command would be evaulated (of course not successfully, but I do not post dangerous commands here). A minimal test is to exclude all strings, which contain '!', 'dos' and 'system', but even the local variables of the CellEditCallback function can be overwritten, 'error' let the program stop, 'quit' closes Matlab etc.
Perhaps converting the string to a sym expression at first is more secure.
