You can’t have a circular reference like that. You have the ‘Initial Content’ of your input field being set by an expression that uses the value of your input field. The suggestion was to have a second text field below your input to show what he username is going to be and then apply the regex AGAIN in the workflow when you save.
This, unfortunately is a sorely lacking feature of Bubble. If you could set the isValid property of an element you could use the regex in a condition and cause the field to be marked invalid and then Bubble would automatically stop it from submitting. I requested this feature here Set "valid" property for conditions