The following validator is provided by catworkx Portfolio Management for Jira:

[cwx-ppm] Project Key Validator (Syntax and Existence)

Validates whether the value of a text field matches a valid issue key respectively whether the project does not exist yet. To do this the project name is tested by a regular expression that is pre-configured in Jira. To change this regular expression, read the Jira documentation of Atlassian.

Restrictions for project keys:

Only letters, numbers and the underscore are allowed to use.
The first character has to be a letter.
All letters have to be upper-case letters of the modern roman alphabet.

Required information:

Key Field: Select the custom field that contains the project key
Name Field: Select the custom field that contains the project name.

[cwx-ppm] Validate Remote Jira Project

Validates that a project name and key do not already exist in a remote Jira.

Required information:

Application Link: Select the application link to the remote Jira.
Run as User: Select the user to access the remote Jira. If the field is empty, the current user is used.
Key Field: Select the custom field that contains the project key
Name Field: Select the custom field that contains the project name.