import re
content = 'Should replace [obj[text]obj] inside [loc[brackets]loc]'
pattern = re.compile(r"\[([A-Za-z0-9]+)(\[(.*?)\])([A-Za-z0-9]+)\]")
new = re.sub(pattern, r'[\1xxx\4]', content);
print(new)
aW1wb3J0IHJlCgpjb250ZW50ID0gJ1Nob3VsZCByZXBsYWNlIFtvYmpbdGV4dF1vYmpdIGluc2lkZSBbbG9jW2JyYWNrZXRzXWxvY10nCnBhdHRlcm4gPSByZS5jb21waWxlKHIiXFsoW0EtWmEtejAtOV0rKShcWyguKj8pXF0pKFtBLVphLXowLTldKylcXSIpCm5ldyA9IHJlLnN1YihwYXR0ZXJuLCByJ1tcMXh4eFw0XScsIGNvbnRlbnQpOwpwcmludChuZXcp