Substitutions and backreference

Regular expressions not only express matching, but they’re also useful for expressing substitutions and replacements. The function re.sub helps you do such a substitution. Here’s the help text:

sub(pattern, repl, string, count=0, flags=0)

Return the string obtained by replacing the leftmost non-overlapping occurrences of the pattern in string by the replacement repl. repl can be either a string or a callable; if a string, backslash escapes in it are processed. If it is a callable, it’s passed the Match object and must return a replacement string to be used.

Here’s an example of how to use it:

What’s happening here? The first argument to re.sub is the pattern to match in the string. The second argument is the replacement, i.e., what do with the parts of the string that match. The third argument is the string to work on. The net effect is to transform "Michael Greenberg" into "Greenberg, Michael". Neat! But what are \1 and \2?

The form \N where N is a number is a backreference. Used primarily for substitutions, backreferences refer to the group numbers from match.group. So \1 refers to the first group of one-or-more word characters (here, "Michael") and \2 refers to the second group of one-or-more word characters (here, "Greenberg"). The rest of the replacement is fixed—you can think of it as a string with a few of these backreferences thrown in.

Backreference is pretty powerful: you can do quite a bit with very little. For example:

Even if you have capturing groups, you have no obligation to use those groups.