7e. Regex embedded code 

• You almost certainly don’t need this

/ ... (?{}) ... /

Warning: experimental

This zero-width assertion executes any embedded Perl
code. It always succeeds, and its return value is set
as $^R.

Inside a (?{...}) block, $_ refers to the string the
regular expression is matching against. You can also
use pos() to know what is the current position of
matching within this string.