How can we perform validation on data input by user in custom fields in Jira using Groovy script?

[Gaurav Bansal]

Client wishes to customize Jira and for that we are supposed to write script that can add custom fields. Script is also supposed to perform validation(restrict custom fields to allow only validated data e.g. Input String should have characters only and length should be between 5 and 15). I know how to add fields using UI but requirement is using Groovy script. Looking forward to any help regarding how to add validated custom fields in Jira using Groovy script. Thanks in advance.