is_basic_expr.py

import re

def is_basic_expr(expr):
    """
    A basic expression has one or more positive integers separated by +-*/.
    """
    #  finds more than one space
    if re.search("\s\s+", expr):
        return False
    # finds negative numbers
    if re.search("-\d+", expr):
        return False
    # make sure ends in digit
    if not re.search("\d+$", expr):
        return False
    # make sure no double operators
    if re.search("(\+|-|\*|\/)\s+(\+|-|\*|\/)", expr):
        return False
    # make sure no digits separated by space
    if re.search("\d+\s\d+", expr):
        return False
    # make sure all characters are valid
    return all([c in "0123456789+-*/ " for c in expr])