validate string is valid maths

[Christian Heimes]

Steven D'Aprano wrote:
> def rationalise_signs(s):
>     while "++" in s or "+-" in s or "-+" in s or "--" in s:
>         s = s.replace("++", "+")
>         s = s.replace("--", "+")
>         s = s.replace("+-", "-")
>         s = s.replace("-+", "-")
>     return s

I assume it's faster to check the length of the string s after each
iteration and compare it to the last iteration. When the string hasn't
changed break out of the loop.