my $entity = join '|', keys %entitylist;
$string =~ s/($entity)/$entitylist{$1}/g;

##</code><code>##

my %entitylist = ( a => 1, b => 2);
my $string = 'abc';
my $entity = join '|', keys %entitylist;
$string =~ s/($entity)/$entitylist{$1}/g;
print $string, "\n";
__END__
12c