The function complex('2-3j') is valid but the function complex('2 - 3j') is invalid.

The complex() function in Python can accept a string argument representing a complex number in the form 'a+bj' or 'a-bj', where a and b are real numbers. In the first case, '2-3j', there are no spaces between the real and imaginary parts, so it is valid. However, in the second case, '2 - 3j', there is a space between 2 and -, making it an invalid input for the complex() function.