#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
    chomp;
    while( s/(?'foo'
		(	
			(?=([[:upper:]]\.\s){2})[[:upper:]\.\s]{2,}
			|
			(?=([[:upper:]]\s){2})[[:upper:]\s]{2,}
			|
			(?=([[:upper:]]\.){2})[[:upper:]\.]{2,}
			|
			[[:upper:]]
		){2,}
	   )//x ) {
	
	my $acronym = $+{foo};
	print qq("$acronym"\n);
    }
}
    
exit(0);

__DATA__
L F and LF and L.F. and L. F. and not L, F.
some HTML some XML.
or X.H.T.M.L. or X. H. T. M. L. or even X H T M L
but not U and I,
or You and I.
...