Spoke too soon, although this might be a valid divergence in behavior:

$ cat test.py 
import re, regex

text = "test: 2"

print regex.sub('(test)\W+(\d+)(?:\W+(TEST)\W+(\d))?', '\\2 \\1, \\4 \\3', text)
print re.sub('(test)\W+(\d+)(?:\W+(TEST)\W+(\d))?', '\\2 \\1, \\4 \\3', text)


$ python test.py 
2 test,  
Traceback (most recent call last):
  File "test.py", line 6, in <module>
    print re.sub('(test)\W+(\d+)(?:\W+(TEST)\W+(\d))?', '\\2 \\1, \\4 \\3', text)
  File "/usr/lib64/python2.7/re.py", line 151, in sub
    return _compile(pattern, flags).sub(repl, string, count)
  File "/usr/lib64/python2.7/re.py", line 278, in filter
    return sre_parse.expand_template(template, match)
  File "/usr/lib64/python2.7/sre_parse.py", line 787, in expand_template
    raise error, "unmatched group"
sre_constants.error: unmatched group