How to Properly Replace Matches with Numbered Groups in Python\'s re.sub?

Python RE.Sub Group: Numbering After Number

When using regular expressions in Python, the re.sub method can be used to substitute matches with a replacement string. However, the syntax can be tricky when working with numbered groups.

A Misinterpreted Attempt:

Initially, you might try:

<code class="python">re.sub(r'(foo)', r'23', 'foobar')</code>

Expecting the output to be 'foo123bar', but instead, it returns 'J3bar', replacing 'foo' with 'J3' for reasons that might not be immediately clear.

The Correct Syntax:

To accurately replace 'foo' with 'foo123', use the following syntax:

<code class="python">re.sub(r'(foo)', r'\g<1>123', 'foobar')</code>

Here's a detailed explanation:

• The regular expression (foo) captures the substring 'foo'.
• The replacement string g<1>123 references the captured substring and appends "123" to it.
• g<1> is a numbered group reference, where '<1>' represents the first captured group.

Documentation Excerpt:

According to the documentation, the syntax of g or g is used to:

In addition to character escapes and backreferences ... g will use the substring matched by the group named name, as defined by the (?P...) syntax. g uses the corresponding group number.

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