Input text field validation

Input text field validation

Good day! Guys.. i need your help again..

i don't understand this structure..

i know its for email validation, but how the line 1 works?
and also if JQuery has this function?

  1.     if (!/^[A-Za-z0-9_\.\-]+\@([A-Za-z0-9\-]+\.)+[A-Za-z0-9]{2,4}$/.test(

            document.forms[0]["email"].value)) {

            alert("Please enter a valid e-mail address.");




            return false;


Thank you very much and God bless you all!