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After a match against some variable $var:
* $` is the same as substr($var, 0, $-[0])
...
* $1 is the same as substr($var, $-[1], $+[1] - $-[1])
* $2 is the same as substr($var, $-[2], $+[2] - $-[2])
* $3 is the same as substr($var, $-[3], $+[3] - $-[3])