If you have a regular expression like this:
([0-9])([a-z])?
matching this string:
1 1a
and replacing with this:
yx
you get what expected:
yx yx

BUT:
If you replace with this:
\1\2
you get nothing replaced, because the group \2 
doesn't exist for the pattern "1".
But it does exist for the pattern "1a"!

We have multiple possibilities here:
1.) The string "1" gives no result, because \2 
doesn't exist. The string "1a" gives a result, so the 
output should be: 1a
2.) The sring "1" gives a result, because \2 is 
handled like an empty string. The string "1a" gives a 
result, so the output should be: 1 1a


I think the case that the sring "1" has no results, 
but effects the string "1a" wich would normaly have a 
result, is bad.

What are your thoughts on it?


Test code:
import re

# common variables

rawstr = r"""([0-9])([a-z])?"""
embedded_rawstr = r"""([0-9])([a-z])?"""
matchstr = """1 1a"""

# method 1: using a compile object
compile_obj = re.compile(rawstr)
match_obj = compile_obj.search(matchstr)

# method 2: using search function (w/ external flags)
match_obj = re.search(rawstr, matchstr)

# method 3: using search function (w/ embedded flags)
match_obj = re.search(embedded_rawstr, matchstr)

# Retrieve group(s) from match_obj
all_groups = match_obj.groups()

# Retrieve group(s) by index
group_1 = match_obj.group(1)
group_2 = match_obj.group(2)

# Replace string
newstr = compile_obj.subn('\1\2', 0)