What is the regex to replace a single space between a closing brace and a letter?

I want to replace all occurrences of

• case 1: a single space between 2 letters
• case 2: a single space between a closing brace } and a letter

with \times such that

'a^{3} + 3 a^{2} b + 3 a b^{2} + b^{3}'

becomes

'a^{3} + 3 \\times a^{2} \\times b + 3 \\times a \\times b^{2} + b^{3}'

I can successfully do the first case with \b\s\b but I don’t know how to do for the second one.

My attempt below just produces

'a^{3} + 3 \\times a^{2}  b + 3 \\times a \\times b^{2} + b^{3}'

where I fail to put \\times between a^{2} and b.

import re
import sympy
from IPython.display import display
a,b = sympy.symbols('a b')

f = (a+b)**3
expr = sympy.latex(f.expand())
display(expr)
expr = re.sub(r'(\b\s\b)|(IDONTKNOW)', r' \\times ', expr)
display(expr) 

>Solution :

Like this?

re.sub(r'(?<=[}\w])\s(?=\w)', r' \times ', expr)

The lookarounds are assertions which do not include the text in the match. (?<=...) requires the text before the main match to be either } or an alphanumeric, and (?=...) requires the text after the main match to be an alphanumeric.

(\w is not exactly "an alphanumeric"; it matches numbers and underscores in addition to alpbabetics. Probably replace it with e.g. [A-Za-z] if you strictly need to match only English alphabetics.)