Backreferences

1. Backreferences

Now you know how to capture parts of a regular expression you are ready to learn how to refer to the captured pieces. Referring to a captured part of a pattern is known as a backreference.

2. Backreferences

Since there may be multiple captures, regular expressions use a simple count from left to right to refer to the different captures. The first capture can be referred to using the REF1 constant in rebus: You can see it translates to a backslash 1 in the regular expression language. The second capture can be referred to using REF2 and so on, up to possible the ninth capture with REF9.

3. In a pattern

Backreferences can be used directly in a pattern, to allow for repeated patterns. This pattern looks for a word by looking for a space then one or more word characters followed by another space.

4. In a pattern

If we capture the word part we can refer to it later and the pattern now picks up repeated words. Backreferences can also be used

5. In a replacement

in the replacement argument to str_replace. The replacement should be a string, so if you want to combine the back reference with some fixed characters, you'll need to combine them with str_c. You could use this to replace the repeated word with a single one. You simply need to replace our match with "a space, then the captured part of the expression"

6. Let's practice!

OK, your turn to practice backreferences.

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