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