my $string_exp = 'something in this string is a number. ${=NumberFormat "99999999"} is a number. And another ${=CharFormat "X*"} is ${=NumberFormat "9+"}.';
# remove the left curly bracket from the last NumberFormat placeholder in the above string
my $placeholder = '${=CharFormat "X*"}';

print "$string_exp\n";
print "$placeholder\n";

if ( $string_exp =~ m/\Q$placeholder\E(.*)?(\$\{)|\z/ ) {
 print "\"$1\"\n";
}

print "END\n";