AngularJS - Remove leading and trailing whitespace from input-box using regex

AngularJS trims the input by default, so you need to use ng-trim="false" in order to pass leading and trailing whitespace to your pattern regex.

See documentation:

ngTrim (optional)

If set to false Angular will not automatically trim the input. This parameter is ignored for input[type=password] controls, which will never trim the input.

(default: true)

Do you want leading/trailing spaces allowed on the whole string, or around each individual address?

For the former your regex should be

/^(\s*([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5}){1,25})+([,.](([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5}){1,25})+\s*)*$/

and for the latter

/^(\s*([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5}){1,25}\s*)+([,.](\s*([a-zA-Z0-9_\-\.]+)@([a-zA-Z0-9_\-\.]+)\.([a-zA-Z]{2,5}){1,25})+\s*)*$/