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 parentheses.	/(?:foo)/ matches, but does not remember "foo" in "foo bar".
|	Alternation combines clauses into one regular expression, and then matches any of the individual clauses. x|y matches either x or y. It is similar to the "OR" statement.	/morning|night/ matches "morning" in "good morning" and matches "night" in "good night".
()\n	"\n" (where n is a number from 1-9) when added to the end of a regular expression pattern, allows you to back reference a subpattern within the pattern, so, the value of the subpattern is remembered and used as part of the matching.	/(no)\1/ matches "nono" in "nono". "\1" is replaced with the value of the first subpattern within the pattern, or (no), to form the final pattern.