If you're dedicated enough to tinker with regular expressions (in the example you're provided) then I suggest just to go one step farther and write a small script - say, in Python. This way you'll be able to perform absolutely any transformations to file names.

I would estimate the python script would be no longer than 15-20 lines long so it's definitely not a huge task. Using just regexps, as you're trying, is much more limited.

Here's my take on such script:

#!/usr/bin/python

import os,re



files = os.listdir('.')



SEASONS = (

 (1, 1, 3), # the format is - season number, first episode, last episode

 (2, 50,52),

 (3, 53,55),

 (4, 56,99),

)



for f in files:

    # skip all files which are not .mp4

    if not f.endswith(".mp4"):

        continue



    # find the first number in the filename

    matches = re.findall("d+", f)

    if not len(matches):

       print "skipping", f

    num = int(matches[0])



    for season in SEASONS:

        if num <= season[2]:

            season_num = season[0]

            ep_num = num - season[1] + 1

            new_file_name = "BleachS%02dE%02d.mp4" % (season_num, ep_num)

            # This is for testing

            print "%s ==> %s" % (f, new_file_name)

            # Uncomment the following when you're satisfied with the test runs

            # os.rename(f, new_file_name)

            break



print "Done"

It looks like I under-estimated the script size (it's 36 lines atm), though I'm sure if you go to stackoverflow with this code, you'll get many suggestions which are much more elegant

And just because I said it can be done in 15 lines... the following is 20 lines, 5 of which is configuration :P

#!/usr/bin/python

import os, re, glob



SEASONS = (

 {'num':1, 'first':1, 'last':3}, # the format is - season number, first episode, last episode

 {'num':2, 'first':50, 'last':52},

 {'num':3, 'first':53, 'last':55},

 {'num':4, 'first':56, 'last':99},

)



files = glob.glob('bleach*.mp4')

for f in files:

    num = int(re.findall("d+", f)[0])  # find the first number in the filename

    for season in SEASONS:

        if num > season['last']: continue

        new_file_name = "BleachS%02dE%02d.mp4" % (season['num'], num - season['first'] + 1)

        print "%s ==> %s" % (f, new_file_name) # This is for testing

        # os.rename(f, new_file_name) # Uncomment this when you're satisfied with the test runs

        break