[Python] Replace letters inside words that start with and end with a specific letter not...

I am using the following code to replace each non-initial and non-final letter with ! inside words starting and ending with a not containing a anywhere else in the words:

import re

txt = 'aba accca azzza wwwwa accca wwasdwaww abmasedmwa'

regex = re.sub(r'\ba([^a]+)a\b', r'!\1!', txt)
print(regex)


However, the output is

!b! !ccc! !zzz! wwwwa !ccc! wwasdwaww abmasedmwa


I'd like to get this output:

a!a a!!!a a!!!a wwwwa a!!!a wwasdwaww abmasedmwa

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