I noticed while reviewing #21069 that a case like this:

f"{1:\x64}"

results in an Err from FormatSpec::parse despite being valid on Python 3.11:

Python 3.11.13 (main, Sep  2 2025, 14:20:25) [Clang 20.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> f"{1:\x64}"
'1'

and also 3.14:

Python 3.14.2 (main, Dec  9 2025, 19:03:28) [Clang 21.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> f"{1:\x64}"
'1'

(and I assume all other versions as well, #21069 was just about the behavior of backslashes in interpolations in 3.11 vs 3.12+).

This seems like quite a niche issue, and you can see that GitHub's syntax highlighting doesn't like it either, but I wanted to open a follow-up issue regardless.