The text matched by the highest used capture group of the last successful search pattern. (See "Scoping Rules of Regex Variables"). It is logically equivalent to the highest numbered capture variable ($1, $2, ...) which has a defined value.
This is useful if you don't know which one of a set of alternative patterns matched. For example:
/Version: (.*)|Revision: (.*)/ && ($rev = $+);This variable is read-only, and its value is dynamically scoped.
Mnemonic: be positive and forward looking.