regex - python re.sub group: number after umber

Question

How can I replace foobar with foo123bar?

This doesn't work:

>>> re.sub(r'(foo)', r'1123', 'foobar')
'J3bar'

This works:

>>> re.sub(r'(foo)', r'1hi', 'foobar')
'foohibar'

I think it's a common issue when having something like umber. Can anyone give me a hint on how to handle this?

Answer 1

The answer is:

re.sub(r'(foo)', r'g<1>123', 'foobar')

Relevant excerpt from the docs:

In addition to character escapes and backreferences as described above, g will use the substring matched by the group named name, as defined by the (?P...) syntax. g uses the corresponding group number; g<2> is therefore equivalent to 2, but isn’t ambiguous in a replacement such as g<2>0. 20 would be interpreted as a reference to group 20, not a reference to group 2 followed by the literal character '0'. The backreference g<0> substitutes in the entire substring matched by the RE.