CircuitPython

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# 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'))