I just noticed that https://docs.python.org/3/library/string.html#formatspec links to the "integer" definition in the main Python grammar for the permitted format of numeric fields.

This isn't accurate:

```
>>> format(10e4, ",.2f")
'100,000.00'
>>> format(10e4, ",.0x2f")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier
>>> format(10e4, ",.2_2f")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier
```

Instead, the `width` and `precision` subfields should be defined as "digit+"

Just submitted a PR for this issue, however I only signed the CLA an hour ago so it hasn't gone through yet.

This is my first contribution, I followed the guide but please let me know if I missed anything. Thanks!

New changeset 8b5fa289fdb04b6b919cf95fa99246aa872e47a8 by Mariatta (nathankerr96) in branch 'master':
bpo-32720: Fixed the definition for width and precision in format mini-language doc (GH-5482)
https://github.com/python/cpython/commit/8b5fa289fdb04b6b919cf95fa99246aa872e47a8

New changeset 133514e9dcea9c8cc9d9dd459b1a7fb502747d7e by Mariatta (Miss Islington (bot)) in branch '3.7':
bpo-32720: Fixed the definition for width and precision in format mini-language doc (GH-5482) (GH-5524)
https://github.com/python/cpython/commit/133514e9dcea9c8cc9d9dd459b1a7fb502747d7e

New changeset 3bd749b2122c17b835dc438cdaef63bfdfd61344 by Mariatta (Miss Islington (bot)) in branch '3.6':
bpo-32720: Fixed the definition for width and precision in format mini-language doc (GH-5482) (GH-5525)
https://github.com/python/cpython/commit/3bd749b2122c17b835dc438cdaef63bfdfd61344

What about arg_name and element_index?

>>> '{}'.format(123)
'123'
>>> '{0}'.format(123)
'123'
>>> '{0x0}'.format(123)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: '0x0'
>>> '{0_0}'.format(123)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: '0_0'
>>> '{0[0]}'.format([123])
'123'
>>> '{0[0x0]}'.format([123])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers or slices, not str
>>> '{0[0_0]}'.format([123])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list indices must be integers or slices, not str

I didn't think to check those - it looks like they have the same problem with the same fix (i.e. the actual syntax is "digit+").

New changeset 7a561afd2c79f63a6008843b83733911d07f0119 by Mariatta in branch 'master':
bpo-32720: Fixed the replacement field grammar documentation. (GH-5544)
https://github.com/python/cpython/commit/7a561afd2c79f63a6008843b83733911d07f0119

New changeset a747cf6d9b87ea8c7ce3a42d09e5b966c8e83fa0 by Mariatta (Miss Islington (bot)) in branch '3.7':
bpo-32720: Fixed the replacement field grammar documentation. (GH-5544) (GH-5546)
https://github.com/python/cpython/commit/a747cf6d9b87ea8c7ce3a42d09e5b966c8e83fa0

New changeset 6708be744d032aeffab3e2cf0b064a41a719d8a1 by Miss Islington (bot) in branch '3.6':
bpo-32720: Fixed the replacement field grammar documentation. (GH-5544) (GH-5547)
https://github.com/python/cpython/commit/6708be744d032aeffab3e2cf0b064a41a719d8a1

Thanks!