# This regex expects the first token to be a valid atomic expression.
# The next token can be a valid atomic expression or an operator
# At least one sequence of two valid atomic expressions and one operator must exist for the postfix to be valid and reduce to a binary postfix primitive
# Finally, we may have any number of atomic expressions and operators... the expression might be found to be invalid at some point in the evaluation process
# It also shouldn't match simple binary expressions- that is left to the other functions. This implies atleast one more atomic expression, either before or after it.
# (Just an extra operator is obviously invalid).