Regex: ?: notation (Question mark and colon notation) [duplicate]

Question

I have the following Java regex, which I didn't write and I am trying to modify:

^class-map(?:(\\s+match-all)|(\\s+match-any))?(\\s+[\\x21-\\x7e]{1,40})$            ^                                 ^ 

It's similar to this one.

Note the first question mark. Does it mean that the group is optional? There is already a question mark after the corresponding ). Does the colon have a special meaning in regex?

The regex compiles fine, and there are already JUnit tests that show how it works. It's just that I'm a bit confused about why the first question mark and colon are there.

Answer 1

(?: starts a non-capturing group. It's no different to ( unless you're retrieving groups from the regex after use. See What is a non-capturing group? What does a question mark followed by a colon (?:) mean?.

Answer 2

A little late to this thread - just to build on ryanp's answer.

Assuming you have the string aaabbbccc

Regular Expression

(a)+(b)+(c)+ 

This would give you the following 3 groups that matched:

['a', 'b', 'c'] 

Regular Expression with non-capturing parenthesis

Use the ?: in the first group

(?:a)+(b)+(c)+ 

and you would get the following groups that matched:

['b', 'c'] 

Hence why it is called "non-capturing parenthesis"

Example use case:

Sometime you use parenthesis for other things. For example to set the bounds of the | or operator:

"New (York|Jersey)" 

In this case, you are only using the parenthesis for the or | switch, and you don't really want to capture this data. Use the non-capturing parenthesis to indicate that:

"New (?:York|Jersey)"