Another way to think of it is that (zero-width) look-around assertions act from positions between characters, so in the string
                             5 4 W
                                ^^
                                ||
    (?=4) looks forward from here|
                                 |
         at the character (W) here

So is W the same as 4? It is not. (Actually, the regex checks every possible position since it is not constrained to do otherwise, but even so, W is never 4.) And for  (?=\d) likewise.

Update: Also consider: 

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '54W';
 ;;
 print 'match' if $s =~ m{ (?=4) . W }xms;
"
match

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In reply to Re: zero-width assertions for extended regular expressions by AnomalousMonk
in thread zero-width assertions for extended regular expressions by johnrcomeau

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