Field value validation
Return "Valid" if the custom field value matches the regular expression and "Invalid" if it does not.
Configuration
Create a Custom smart text field and use the General parsing mode.
Expression
%{matches(%{issue.cfnnnnn},"6|7|8|9") ? "Valid":"Invalid"}
The values entered as arguments in the matches() parser function can be replaced with different values or with a regular expressions to validate differently the value of the custom field.
Please, note that it is necessary to replace nnnnn with the ID of the custom field.
Used parser functions
The links lead to the JWTC documentation because the parser functions are shared functionalities.