Делаю
print(re.sub("US([0-9])([0-9][0-9][0-9])([0-9][0-9][0-9])","US \1\2\3","In US1234567 we can notice..."))
In US [здесь идут квадраты] we can notice...
print(re.sub("US([0-9])([0-9][0-9][0-9])([0-9][0-9][0-9])","US \1\2\3","In US1234567 we can notice..."))
In US [здесь идут квадраты] we can notice...
re.sub('\d+',lambda m: ' {:,}'.format(int(m.group(0))) ,"In US1234567 we can notice...") Out[57]: 'In US 1,234,567 we can notice...'