I think that's going to start doing the wrong thing if Dr van Houten starts putting letters after his name; the first set are ok but as you start adding more you get unwanted dots.

$ perl -le '$_ = q{Dr P J van Houten MD};
> s[(?<=[A-Z])(?=\s)][.]g;
> print;'
Dr P. J. van Houten MD
$ perl -le '$_ = q{Dr P J van Houten MD FRCS};
> s[(?<=[A-Z])(?=\s)][.]g;
> print;'
Dr P. J. van Houten MD. FRCS
$
[download]

A possible solution is to use alternation of two look-behinds.

$ perl -le '$_ = q{Dr P J van Houten MD FRCS};
> s{(?:(?<=\A[A-Z])|(?<=\s[A-Z]))(?=\s)}{.}g;
> print;'
Dr P. J. van Houten MD FRCS
$
[download]

Cheers,

JohnGG