I came across a scenario where i implemented a custom validation on Sitecore 9 Forms. Sharing my knowledge here. You can achieve this in three simple steps.
Step 1: Create custom validator
Create a custom validator under
/sitecore/system/settings/forms/validation based on Validation(sitecore/templates/System/Forms/Validation) template like below
Step 2: Fill out the fields
Fill out your own regex and error message. You can copy the Type value from other validator.
Step 3: Assign the custom validator
You can now add the new custom validator to any Form fields(
/sitecore/system/Settings/Forms/Field Types). All set and no code required!
Hope you found this blog helpful. Any questions, please leave a comment.