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$window_size--;
my( $windowed_text )= $text_body =~
    /^\s*(?:\S+\s+){$window_start}(\S+(?:\s+\S+){0,$window_size})/;

$_ = q/01234/;
/^..{0}/;    # matches '0'
/^..{0,0}/;  # matches '0'
/^..{0,-1}/; # doesn't match at all