As illustrated here and detailed in “this question”, the function fnmatch() is involved to interpret glob patterns, which means regular expressions are not supported.
This is what gitignore man page mentions:
Otherwise,
git treats the pattern as a shell glob suitable for consumption by fnmatch(3)with theFNM_PATHNAMEflag: wildcards in the pattern will not match a/in the pathname.
For example, “Documentation/*.html” matches “Documentation/git.html” but not “Documentation/ppc/ppc.html” or “tools/perf/Documentation/perf.html“.
You can see some correspondence between glob patterns and regular expressions in those questions.