Question: Optional Regular Expression Grouping

[galyle]

HI, I've looked through this forum, but I haven't been able to find a
resolution to the problem I'm having (maybe I didn't look hard enough
-- I have to believe this has come up before).  The problem is this:
I have a file which has 0, 2, or 3 groups that I'd like to record;
however, in the case of 3 groups, the third group is correctly
captured, but the first two groups get collapsed into just one group.
I'm sure that I'm missing something in the way I've constructed my
regular expression, but I can't figure out what's wrong.  Does anyone
have any suggestions?

The demo below showcases the problem I'm having:

import re

valid_line = re.compile('^\[(\S+)\]\[(\S+)\](?:\s+|\[(\S+)\])=|\s+[\d\
[\']+.*$')
line1 = "[field1][field2] = blarg"
line2 = "    'a continuation of blarg'"
line3 = "[field1][field2][field3] = blorg"

m = valid_line.match(line1)
print 'Expected: ' + m.group(1) + ', ' + m.group(2)
m = valid_line.match(line2)
print 'Expected: ' + str(m.group(1))
m = valid_line.match(line3)
print 'Uh-oh: ' + m.group(1) + ', ' + m.group(2)