micropython/tests/extmod/ure_sub_unmatched.py

# test re.sub with unmatched groups, behaviour changed in CPython 3.5

try:
    import ure as re
except ImportError:
    try:
        import re
    except ImportError:
        print("SKIP")
        raise SystemExit

try:
    re.sub
except AttributeError:
    print("SKIP")
    raise SystemExit

# first group matches, second optional group doesn't so is replaced with a blank
print(re.sub(r"(a)(b)?", r"\2-\1", "1a2"))