Replace named group in regex match

Question

I have the following regular expression:

pattern = '^[a-zA-Z0-9-_]*_(?P<pos>[A-Z]\d\d)_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$'

that matches file names like these:

filename = '151006_655866_Z01_T0001F015L01A02Z01C03.tif'

with groups:

m = re.match(pattern, filename)
print m.group("pos")  # Z01
print m.group("fID")  # F015
print m.group("zID")  # Z01

How can I replace only a specified group with a given string in Python?

I tried to use re.sub with a function call, but don't know how this function should look like:

def replace_function(matchobj):
    # how to replace only a given match group?
    # (the following replaces *all* occurrences of "Z01" in this example)
    return matchobj.group(0).replace(matchobj.group("slice"), "---")

print re.sub(pattern, replace_function, filename)

My desired result would be:

151006_655866_Z01_T0001F015L01A02---C03.tif

Answer 1

You can do what you need using a closure and the start/end index of the chosen matching group:

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)

Output:

151006_655866_---_T0001F015L01A02Z01C03.tif
151006_655866_Z01_T0001---L01A02Z01C03.tif
151006_655866_Z01_T0001F015L01A02---C03.tif
151006_655866_Z01_T0001F015L01A02Z01---.tif

An online implementation is available here