[BUG] dashes not supported in environment variable names in .env files

[vvuk]

Description

This was seemingly fixed in #9570 last year, and looking at the code it doesn't look like the code regressed, but I can't seem to include environment variables with a - in them via env_file in a compose file.

Steps To Reproduce

foo.env contains (hello ASP.NET config env vars):

services__storage-service__http__0=http://storage:2227

docker-compose.yaml contains:

services:
  storage:
    image: ..
    env_file: foo.env

trying to do up errors out with:

failed to read ...foo.env: line 1: unexpected character "-" in variable name "services__storage-service__http__0=http://storage:2227"

(I'm also a little confused why it prints the entire line as "variable name" but I think that's just a printing issue?)

I can define variables with a - in them fine directly in environment: entries in the compose file.

Compose Version

Docker Compose version v2.29.7, Ubuntu 22.04.

Docker Environment

lient: Docker Engine - Community
 Version:    27.3.1
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.17.1
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.29.7
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Anything else?

No response

[vvuk]

Actually, wait. #9570 claims it was fixed in compose-spec/compose-go#336 but the source change (which is the same that remains today):

https://github.com/compose-spec/compose-go/blob/58f8cade1fa80ca0d59947c1ebd965c4c215ab71/dotenv/parser.go#L122-L132:

		case '_', '.', '-', '[', ']':
		default:
			// variable name should match [A-Za-z0-9_.-]
			if unicode.IsLetter(rune) || unicode.IsNumber(rune) {
				continue
			}


			return "", "", inherited, fmt.Errorf(
				`line %d: unexpected character %q in variable name %q`,
				p.line, string(rune), strings.Split(src, "\n")[0])
		}

the patch added '-' to the case statement, but it would have fallen through default anyway -- and the following lines are still checking IsNumber or IsLetter exactly like before. So I'm not sure if this actually did anything to allow -?

[ndeloof]

we had to revert this "feature" as allowing - in variable name conflicts with the "default value" notation (${VAR-default_value}) and makes parsing non-deterministic.

Also, dash in variable name is only supported by some implementation:

IEEE Std 1003.1, 2004 Edition :: 8.1:

Environment variable names used by the utilities in the Shell and Utilities volume of IEEE Std 1003.1-2001 consist solely of uppercase letters, digits, and the '_' (underscore) from the characters defined in Portable Character Set and do not begin with a digit. Other characters may be permitted by an implementation; applications shall tolerate the presence of such names

[andrasballai-guidance]

I think this should be reconsidered. Either require an explicit second character, like :- to refer to default value if more than one - is in an expression, or allow a way to escape a dash