Try for ""abcd\\efgh"":
It matches!
$1: abcd\\efgh
$2: $2 is NOT defined

Try for ""abcd\""":
It matches!
$1: abcd\"
$2: $2 is NOT defined

Try for ""abcd\"efgh"":
It matches!
$1: abcd\"efgh
$2: $2 is NOT defined

Try for ""abcd\\\"efgh"":
It matches!
$1: abcd\\\"efgh
$2: $2 is NOT defined

Try for ""abcd\\i\"efgh"":
It matches!
$1: abcd\\i\"efgh
$2: $2 is NOT defined

Try for ""abcd\\"":
It matches!
$1: abcd\\
$2: $2 is NOT defined