[SOLVED] Problem with Validate As

Problem with Validate As
I need some help, please

In a text field of length 20, only a-z, A-Z and spaces should be allowed.

I can't do it despite the instructions.
(https://xlinesoft.com/phprunner/docs/validation_types.htm)

What do I have to enter in Regexp: and what should the JS file look like.
What or must something else be set in OnPageLoad?

I sit here all night and don't understand.
The example with the IP address did not help me.

Thanks for your help
Andy