It's by design. There's an implicit local for match variables in blocks.

And re "reliability": a regexp that doesn't match will not affect the match variables, they will not (necessarily) be undef, but the value they got on the last succesful regexp — with the above caveat.

In reply to Re^2: The value of pattern match variable $& by bart
in thread The value of pattern match variable $& by jesuashok

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