re.sub() raise exception if no match to substitute

[Jeff Kowalczyk]

I'm trying to keep to an exception-driven style in a data-munging script I'm writing, but
the re.sub() function does nothing and does it silently in the absence of a match to
substitute against. I've elected to use a few sequential regexps to pre-process and
normalize the data before finally splitting it into lists of strings for more structured
processing. Here's one of them, with two test cases:

s1 = r",54321,05022003,..."
s2 = r"1234,54321,05022003,..."
snew1 = re.sub(r"^,",'',s1)
snew2 = re.sub(r"^,",'',s2)

very occasionally there will be a value in the first field before the comma (s2), causing
the match to fail. I'd like to raise an exception to kick off some code handle it.

is there any elegant way to do this kind of thing without a (re.match->if no
groups->raise->else sub) pattern? This particular case is admittedly very simple, but I'm
interested in the general style of raising exceptions for regexp non-matches...