Here is a regex that should work. Note that I made a change in your LaTeX expression because I think there was an error (* sign).
astr="\frac{(\ChebyU{\ell}@{x})^2}{\frac{\pi}{2}}"
import re
pat = re.compile('\\frac\{(.*?)\}\{\\frac\{(.*?)\}\{(.*?)\}\}')
match = pat.match(astr)
if match:
expr="\\frac{{{0}*{2}}}{{{1}}}".format(*match.groups())
print expr
NB: I did not include the spaces in your original expression.
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solved Using Python Regex to Simplify Latex Fractions