$string =~ m/(?:(?=.*?X)X|(?!.*?X))(\S+)/;

##</code><code>##

$string1="abcX123";
$string2="abc123";

$string1 =~ m/(?:(?=.*?X)X|(?!.*?X))(\S+)/;
print "$1\n";

$string2 =~ m/(?:(?=.*?X)X|(?!.*?X))(\S+)/;
print "$1\n";

##</code><code>##

123
abc123