# A quick note on explaining a regex: raw string, grouping reference

I was reading a thread regarding a solution to algorithm problem online, and there was one person asking what r'\1-' mean.

There was one answer to it right down the question, but it didn’t touch many details. So I shared my explanation to it in more detail. Since I am running a blog series on regex, so I thought it would make sense to move my explanation here to my blog under this series.

The problem:

  What is

r’\1-’

?


My explanation:

There are two syntax sugar here:

1. First, r at the beginning denotes that it is a raw string, which tells python not to interpret special sequence prefixed with backslash like: \w (match white space), \W(match non-whitespace), \d(match decimal digit, equivalent to 0-9), etc. Therefore, r'\w', represent a regex that match a backslash followed by a w character.
2. Second, \1 denotes a reference previously matched group. Simply, a group is a matched pattern coped with (). with \1, you reference it for future use. Pls do note that it has to be outside character class to be effective (it cannot be inside square brackets [], which denotes a character class in regex).
3. -, is just matching a hyphen(-)

Reference(s):

1. Here is the link to orignal thread I was reading: https://leetcode.com/problems/license-key-formatting/discuss/96511/Python-solution-based-on-regex/119946?page=1
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