How to Use Group Numbers in Regular Expression Substitution in Python?

Replacement with Group Number in Regular Expression Substitution

When attempting to replace a placeholder with a group number followed by a specified string, the statement re.sub(r'(foo)', r'1123', 'foobar') fails, resulting in 'J3bar'. This is because the numeric portion is interpreted literally and not as a backreference to the group.

To resolve this, one can utilize the g syntax, where group_number denotes the group to be referenced. For example, re.sub(r'(foo)', r'g<1>123', 'foobar') effectively replaces 'foobar' with 'foo123bar'.

The Python documentation explains this functionality as follows:

• g references the substring matched by a group named 'name'.
• g references the corresponding group number.
• g<0> references the entire substring matched by the regular expression.

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