python regex: either or or both with separator

Question

I need a regular expression to match a, b or a;b.

I cannot write a|b|a;b because a and b contain named groups and if I try to do this I get an Exception: redefinition of group name 'a' as group 8; was group 3 at position 60.

a;?b does not work either because ab must not be matched.

How would you solve this? Is this possible with the re library? I have heard there is also a library called pyparsing. Would that be better suited for this problem?

---

Background: This is a follow up question to this one. Because it does not seem to be possible to pass through color codes in urwid or curses I am trying to decode the color codes I am getting from git so that urwid can reencode these colors.

To avoid problems with copy & paste I am leaving out the leading control character in the following regular expressions:

Working regex, except that it does not match [1m (bold) which is used in a test program:

reo_color_code = re.compile(
    r'\['
    r'((?P<series>[01]);)?'
    r'((?P<fgbg>[34])(?P<color>[0-7]))?'
    r'm'
)

Not compiling regex:

reo_color_code = re.compile(
    r'\['
    r'('
        r'((?P<series>[01]))'
        r'|'
        r'((?P<fgbg>[34])(?P<color>[0-7]))'
        r'|'
        r'((?P<series>[01]));((?P<fgbg>[34])(?P<color>[0-7]))'
    r')'
    r'm'
)

Throws the exception

re.error: redefinition of group name 'series' as group 8; was group 3 at position 60

Answer 1

What I'd do in this case wouldn't be try to build a single regex to solve the entire problem, instead I'd implement a method like the following (also using re but at different levels):

def get_info(s):
    if s.startswith('[') and s.endswith('m'):
        p = s[1:-1]
        if ';' in p:
            m = re.match('^([01]);([34])([0-7])$', p)
        else:
            m = re.match('^([01])$|^([34])([0-7])$', p)
        if m:
            return tuple(m.groups())
    return None, None, None

You can use it like:

>>> serie, fgbg, color = get_info('[1;37m')
>>> serie, fgbg, color
('1', '3', '7')

PS: Didn't do too many tests. Hope it helps.