Grouping, alternation, and back reference

In the following table, you can find the patterns for grouping, alternation, and back reference. The grouping is used to group a set of characters in a Regex. The alternation is used to combine characters into a single regular expression, and the back reference is used to match the same text as previously matched by a capturing group:

Pattern	Description	Example
(x)	This groups characters together to create a clause, that is, it matches x and remembers the match. These are called capturing parentheses.	/(foo)/ matches and remembers "foo" in "foo bar".
()	Parenthesis also serves to capture the desired subpattern within a pattern.	/(\d\d)\/(\d\d)\/(\d\d\d\d)/ matches "12", "12", and "2000" in "12/12/2000".
(?:x)	This matches x but does not capture it. In other words, no numbered references are created for the items within the parenthesis. These are called non-capturing...