Regex groups

Parentheses are used to enclose groups

/(ex)amp(le)/

They are good for expression ORs

/(tr|kn)ee/ — will match both 'tree' and 'knee'

Good for capturing parts of strings using groups. Here’s a ruby example.

string = "example"
one, two = string.match(/(ex)amp(le)/).captures
=> ["ex", "le"]

Referring back to a group

([abc])\1

\1 matches the same part that was matched by the 1st group

"foobarfoo".match(/(foo).*(r\1)/).captures
=> ["foo", "rfoo"]

Also works with \2, \3 and so on.

To ignore a group (don’t want to capture but only want to OR)

/(?:c|d)ar(t)/ — will not capture 'c' or 'd' but will capture last 't'