Replace named group with re.sub

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Python

regular-expression

Hi,
i am trying to replace a named group with another one but cannot figure out how to specify the first parameter in re.sub function:

reegex = re.compile("^big dog.*?(?P<groupname>big cat.*?));", re.DOTALL|re.MULTILINE)

res = re.sub(reegex, r'\g<groupname>' + "HELLO", fileData)

How can i replace only the named group part?

Regards

What I have tried:

Also tried to write it like this:
res = re.sub(r'\g<groupname>', r'\g<groupname>' + "HELLO", fileData)