Adding Workflow Field Validation in Sitecore 9.1 and above

If you want to enable the field validation on the Item with Workflow.

1)  Create/Use Validation rule under 

/sitecore/system/Settings/Validation Rules/Field Rules

on the you want to validate.

I have added as below.

2) 1st three validators will work without any other effort. 

for workflow you have to create a Workflow Action Item under that any workflow state(in my case Draft) and Commands as below using template :  /sitecore/templates/System/Workflow/Validation Action

3) Configure the Validate action as below

Type: Sitecore.Workflows.Simple.ValidatorsAction, Sitecore.Kernel

Max Result Allowed: Warning,