you can add validation on the page,microflow and in domain model
Refer the below documentation
Also, check the below forum link
I’d suggest validating user input using a Regex. You can either whitelist allowed symbols or blacklist disallowed symbols. The most failsafe method is probably to whitelist all characters that you allow in your input field.
You will need to make a microflow for this validation.
For example, you press save and the microflow checks via a Regex if there isn't an emoji in the string. If not, then you commit the object and show the next screen. If there is, you can show a validation message and end the flow. You can use this Regex:
I have not tested these XPath's so make sure to do so yourself.