The following validator is provided by catWorkX JIRA Portfolio Management:

[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:

Field that contains the project key
Field that contains the project name.