import re
from functools import partial
 
pattern = '^[\w-]*_(?P<pos>[A-Z]\d{2})_T\d{4}(?P<fID>F\d{3})L\d{2}A\d{2}(?P<zID>Z\d{2})(?P<cID>C\d{2})\.tif$'
filename = '151006_655866_Z01_T0001F015L01A02Z01C03.tif'
 
 
def replace_closure(subgroup, replacement, m):
    if m.group(subgroup) not in [None, '']:
        start = m.start(subgroup)
        end = m.end(subgroup)
        return m.group()[:start] + replacement + m.group()[end:]
 
subgroup_list = ['pos', 'fID', 'zID', 'cID']
replacement = '---'
 
for subgroup in subgroup_list:
    print re.sub(pattern, partial(replace_closure, subgroup, replacement), filename)
 

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