in reply to Lookahead/Lookbehind Regular Expression...

I wouldn't bother with look-ahead/behind on this problem:
$_ = 'u.s. asdf.g. and i. M.C. Escher and P. Picasso'; s/\b(\w)\.(\w)\./$1$2/g; # $_ eq 'us asdf.g. and i. MC Escher and P. Picasso'