How to count characters of a field in Nintex form

I have a Nintex form in SP 2016. Wondering if there is a built in function that will tell me the occurrence of character in a control.

Example:

I have a calculated field that has "#" symbol(s). I want to know how many symbols exists in that field. Based on that I shall perform some validation.

Note: I cannot use workflows to do the validation. This is because I want the form to warn me before I execute a workflow (users clicks on submit button) or leave the form.

Any help will be appreciated.

Thanks.

Mayank