python re.sub group: number after \number

python regex numbers regex-group
zhigang picture zhigang · May 12, 2011 · Viewed 81.8k times · Source

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 \number. Can anyone give me a hint on how to handle this?

Answer

John Gaines Jr. picture John Gaines Jr. · May 12, 2011

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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