Well, MRE doesn't say that those regexen are to be used to find integers -- in fact, it states: 'We don't know what these might be used for ...'.

However, if you do want to find (and extract) just integers you could employ both negative look-ahead, and negative look-behind (lookhind wasn't available when MRE was written):

my $numbers="1 1.0 3.547 123 92.34 343.2234";
while($numbers=~/(?<![\d.])(\d+)(?![\d.])/g){
   print "$1 is an integer\n";
}
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That says: match a bunch of digits that are not preceded by a digit or a dot, and are not followed by a digit or a dot (there be other particular exceptions that I'm not considering at the moment such as grabbing any leading + or - sign if present, but this gives you something to play with).

Update: Ahh, and an important exception I neglected is caught by mirod's version: namely, an integer could occur at the end of a sentence and be legitimately followed by a dot, thus we may want to allow for a trailing dot as long as it isn't followed by digits by changing the negative look-ahead above to the one mirod uses: (?!\d*\.\d).