Summary

re.Match.expand() is broken: it raises AttributeError: module 're' has no attribute '_expand'.

import re
m = re.match(r"(a)(b)", "ab")
m.expand(r"\2\1")
# AttributeError: module 're' has no attribute '_expand'

CPython 3.14.2 returns 'ba'. Verified on main @ 7044fdc.

Cause

crates/vm/src/stdlib/_sre.rs:706 delegates to a re._expand helper:

fn expand(zelf: PyRef<Self>, template: PyStrRef, vm: &VirtualMachine) -> PyResult {
    let re = vm.import("re", 0)?;
    let func = re.get_attr("_expand", vm)?;
    func.call((zelf.pattern.clone(), zelf, template), vm)
}

re._expand existed in older CPython, but CPython 3.12 removed it when template expansion moved to the compiled-template mechanism (_sre.template() + re._compiler._compile_template). Since Lib/re/ is now the 3.14 copy, the helper no longer exists and every Match.expand call fails.

Options:

• implement _sre.template(pattern, template) and the compiled-template fast path like CPython 3.12+, and route Match.expand through re._compiler._compile_template (this also makes re.sub with string templates faster and is needed for full parity), or
• as a minimal fix, call re._compiler._compile_template(pattern, template) directly from Match.expand and apply it.

Lib/test/test_re.py::test_expand covers this but is currently marked @unittest.expectedFailure # TODO: RUSTPYTHON (test_re.py:729), so the regression is invisible in CI; the marker can be dropped once fixed.

Found while running synthetic benchmark/compat suites; researched and reported by Claude on behalf of @youknowone.