[docs] Bug in re example

[Herman L. Jackson]

Hi,

When running the tokenize example at the bottom of http://docs.python.org/py3k/library/re.html#match-objects
I received the following error:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "<stdin>", line 23, in tokenize
    NameError: global name 'val' is not defined

The problem is that val is never assigned to before this line:

    if typ == 'ID' and val in keywords:

Here is a suggested rewrite of the that section:

    elif typ != 'SKIP':
        val = mo.group(typ)
        if typ == 'ID' and val in keywords:
            typ = val
        yield Token(typ, val, line, mo.start()-line_start)

Attached is a complete working example.
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