Relax `alphanum_ext_string` schema to support numeric and version-based environment names

[MagicMyles]

Currently, ReFrame environment names are validated against the alphanum_ext_string definition in the internal JSON schema. This regex explicitly requires names to start with a letter.
https://github.com/reframe-hpc/reframe/blob/develop/reframe/schemas/config.json#L379

This prevents users from using standard versioning or date-based strings as environment names.

Proposed Fix:
Update the alphanum_ext_string pattern in the ReFrame schema definitions to allow digits at the start of the string and include the dot character.

Current Regex:
^([a-zA-Z_]([a-zA-Z0-9_]|-)*)$

Proposed Regex:
^([a-zA-Z0-9_]([a-zA-Z0-9_\.-])*)$

[vkarak]

Hi @MagicMyles, could you give an example of an environment name that would be more natural if it started with a digit?

[MagicMyles]

Hi @vkarak, thanks for helping. Its very appreciated.

Yes we have date based environment names such as 2026.04 which is not allowed due to the alphanum_ext_string pattern check:

"alphanum_ext_string": {
            "type": "string",
            "pattern": "([a-zA-Z_]([a-zA-Z0-9_]|-)*)"
        }

I would say its not a major blocker as we can rename our environments. However it creates inconsistencies which is not ideal.