Email validation using Java Script

function validate()
{
var emailformat = /^[a-zA-Z0-9._-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,4}$/ //regular expression defining a 8 digit number
var email = document.getElementById('<%=txtEmail.ClientID %>').value;
if (email == "") {
alert('Enter emailid...');
return false;
document.getElementById('<%=txtEmail.ClientID %>').focus();
}
if (email.search(emailformat) == -1) { //if match failed
alert("please enter valid emailid...");
return false;
document.getElementById('<%=txtEmail.ClientID %>').focus();
}
}
Call this function in OnClientClick of Button like

OnClientClick="javascript:return validate();"