P0 = [ 1 ; 1 ; 0 ] ;

r0 = [ 0 ; 0 ; 0 ] ;
v  = [ 1 ; -1 ; 0 ] ;
r  = @(l) r0 + l*v ;

% Metodo A
lambda = (P0-r0)' * v / ( v'*v ) ;
dist_A = norm( P0 - r(lambda) )

% Metodo B
e_v     = v / norm(v) ;
d       = P0 - r0 ;
d_v     = dot(d, e_v) * e_v ;
d_ortho = d - d_v;
dist_B  = norm( d_ortho )

% Metodo C (esagerato)
I3 = eye(3,3) ;
M  = [ I3 v ; ...
       v' 0 ] ;
d = P0 - r0 ;
b = [ d ; 0 ] ;
x = M\b ;
d_ortho = x(1:3) ;
dist_C = norm( d_ortho ) 

% Metodo D
d  = P0 - r0 ;
dd = dot(d, d) ;
e_v = v / norm(v) ;
L_v = dot(d, e_v)^2 ;
dist_D = sqrt( dd - L_v )