import re
p = re.compile(r'[-+]?[0-9]*\.?[0-9]+(?:[eE][-+]?[0-9]+)?')
strs = ["4x5x6", "7.2volt", "60BTU", "20v", "4*5", "24in"]
print([p.sub(r" \g<0> ", x).strip() for x in strs])