Regex partial fullmatch fails on negative lookeahead

[Shoooooon]

Bug report

Bug description:

The regex.fullmatch method has incorrect behavior when the partial flag is set and the regex uses negative lookahead:

import regex

a = regex.compile(r"(?!(True|False)\b)(.*)")

print(a.fullmatch("True", partial=True))
# Should produce a match (since "True" is a prefix of the match "Truest") but does not.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[StanFromIreland]

Note that you are using a third party module.

As for the code, your regex fails because partial=True only allows a match to be incomplete at the end of the string, but your negative lookahead fails at the start, since "True" already matches the disallowed pattern (True|False)\b.