Steps to replicate the issue (include links if applicable):

• Please see this diff

What happens?:
When publishing a new Python implementation containing the regular expression [a-zA-Z0-9]+, the “Z0” is replaced by the identifier of the new implementation.

What should have happened instead?:
No substitution, or a warning that substitution will occur.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):

Other information (browser name/version, screenshots, etc.):

Per the diff, a subsequent edit successfully resolves the substitution.