import re

original = r'1234'
substitution = r'\1$'
myre = re.compile('34')

# ^^^ lines i cannot change

# re.error: invalid group reference 1 at position 1
print(myre.sub(substitution, original))

# result: 12\1\$
print(myre.sub(re.escape(substitution), original))

# desired result: 12\1$