(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.

/(dd)/(dd)/(dddd)/ 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".

()

" " (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.