How to Insert a Number After a Group Match using `re.sub()` in Python?

Inserting a Number After a Group Match in Python re.sub

In Python, the re.sub() function allows for replacing substrings in a given string. A common challenge encountered is how to replace a group match with another group followed by a specific number.

To illustrate, consider the task of replacing "foobar" with "foo123bar". Attempting this using a simple expression like re.sub(r'(foo)', r'1123', 'foobar') unfortunately results in "J3bar" instead.

The key to solving this issue lies in employing named capture groups, which enable referencing group matches by names assigned within the regular expression. Using the syntax (?P...), we can define a named capture group for the substring to be replaced.

To achieve the desired replacement, we need to use the special syntax g to refer to the named capture group within the replacement string. Here, we use g<1> to refer to the named capture group "foo."

The corrected regular expression would be:

<code class="python">re.sub(r'(?P<foo>foo)', r'\g<foo>123', 'foobar')</code>

With this expression, the replacement correctly inserts "123" after the group match "foo," yielding the expected output "foo123bar."

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