The regular expression used for matching numbers in the documentation for the regular expressions module (the tokenizer section) doesn't match the string ".5", but does match the string "3.".

Here's a link to the tokenizer section of the documentation: https://docs.python.org/3/library/re.html#writing-a-tokenizer

The tokenizer example uses r'\d+(\.\d*)?' for matching numbers. I would personally match ".5" as a number before I would match "3." as a number. In order to do this, I would use r'(\d*\.)?\d+' instead of r'\d+(\.\d*)?'. Python 3's interpreter matches both "3." and ".5" as numbers when interpreting code, so you could use a different regex example for matching both if you wanted to be consistent with Python's own interpreter.