python re.sub group: number after \number

by

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.

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