Recursion

Recursion matches the whole regex again at a particular point inside the regex, which makes it possible to match balanced constructs.

Subroutine Calls

Subroutine calls allow you to write regular expressions that match the same constructs in multiple places without having to duplicate parts of your regular expression.

Infinite and Endless Recursion

Infinite recursion occurs if there is a path through the regex that allows it to recurse without having matched anything. Endless recursion occurs if every path through the regex requires it to recurse. Regex engines handle these situations differently (or not at all which causes a crash upon infinite recursion).

Quantifiers on Recursion

Quantifiers can be used on recursion but may not work the way you might expect.

Capturing Recursion and Subroutines

Capturing groups inside recursion and subroutine calls are handled differently by the regex flavors that support them.

Backreferences with Recursion Level

Special backreferences match the text stored by a capturing group at a particular recursion level, instead of the text most recently matched by that capturing group.

Backtracking after Recursion and Subroutines

The regex flavors that support recursion and subroutine calls backtrack differently after a recursion or subroutine call fails.