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$LAST_PAREN_MATCH
$+

The text matched by the highest used capture group of the last successful search pattern. 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 dynamically-scoped.

Mnemonic: be positive and forward looking.