Non-capturing parentheses

Parentheses are used to capture backreferences and also to group strings together (as in the regex (one|two|three)). This means that any grouping using parentheses also creates a backreference. Sometimes you want to avoid this and not create a backreference. In this case, non-capturing parentheses come in handy. In the example we just saw, the following would group the words, but would not create a backreference:

(?:one|two|three)

The construct (?: ) is what is used to create a non-capturing set of parenthesis. To further show the difference between the two, consider the following regex:

It is (?:hard|difficult) to say goodbye to (.*)

When matched against the string It is hard to say goodbye to you, this will create a single backreference, which will contain the string you. The first, non-capturing parentheses also allow strings beginning with It is difficult to say goodbye to match, but they do not create a backreference.

Non-capturing parentheses are sometimes referred to as "grouping-only parentheses".