Concatenate a string with a variable

I'm trying to do a rather simple task in Terraform and it's not working:

tfvars:

hosted_zone       = "example.com"
domain            = "my.${var.hosted_zone}"

route_53_record:

resource "aws_route53_record" "regional" {
  zone_id = "${data.aws_route53_zone.selected.zone_id}"
  name    = "${var.domain}"
  type    = "A"
  ttl     = "300"
  records = ["4.4.4.4"]
}

When I run terraform plan I'm getting this:

+ aws_route53_record.regional
      id:                 <computed>
      allow_overwrite:    "true"
      fqdn:               <computed>
      name:               "my.${var.hosted_zone}"
      records.#:          "1"
      records.3178571330: "4.4.4.4"
      ttl:                "300"
      type:               "A"
      zone_id:            "REDACTED"

domain should be my.example.com. How do I join the variable hosted_zoned and a string to form domain?

You can't use interpolation in a tfvars file.

Instead you could either join it directly in your Terraform like this:

terraform.tfvars

hosted_zone = "example.com"
domain      = "my"

main.tf

resource "aws_route53_record" "regional" {
  zone_id = data.aws_route53_zone.selected.zone_id
  name    = "${var.domain}.${var.hosted_zone}"
  type    = "A"
  ttl     = "300"
  records = ["4.4.4.4"]
}

Or, if you always need to compose these things together you could use a local:

locals {
  domain = "${var.domain}.${var.hosted_zone}"
}

resource "aws_route53_record" "regional" {
  zone_id = data.aws_route53_zone.selected.zone_id
  name    = local.domain
  type    = "A"
  ttl     = "300"
  records = ["4.4.4.4"]
}