Read directory listing of files and parse out the highest number?

[cubangt]

new detail

So it would def need to look at ONLY files that start with igMess and contain 2 underscores ( _ )
seems we have older files that are formatted like this:
igMess_1.csv
igMess_2.csv

The newer format is:
igMess_1_.csv
igMess_2_.csv

So only care to look at these and return the next highest number.

I have this sample script im playing around with, but not sure how to only consider the files with 2 of those underscores.

# iterate over files in
# that directory
for filename in os.listdir(directory):
    f = os.path.join(directory, filename)
    d = filename
    # checking if it is a file
    if d.startswith("igMess_"):
        print(d)