function emailCheck (form, fieldname) {
var emailStr = document.getElementById(fieldname).value;

/* The following variable tells the rest of the function whether or not
to verify that the address ends in a two-letter country or well-known
TLD.  1 means check it, 0 means don't. */
                                                                                                                                             
var checkTLD=1;
                                                                                                                                             
/* The following is the list of known TLDs that an e-mail address must end with. */
                                                                                                                                             
var knownDomsPat=/^(sk|tv|sx|com|net|org|edu|int|mil|gov|arpa|biz|aero|name|coop|info|pro|museum)$/;
                                                                                                                                             
/* The following pattern is used to check if the entered e-mail address
fits the user@domain format.  It also is used to separate the username
from the domain. */
                                                                                                                                             
var emailPat=/^(.+)@(.+)$/;
                                                                                                                                             
/* The following string represents the pattern for matching all special
characters.  We don't want to allow special characters in the address.
These characters include ( ) < > @ , ; : \ " . [ ] */
                                                                                                                                             
var specialChars="\\(\\)><@,;:\\\\\\\"\\.\\[\\]";
                                                                                                                                             
/* The following string represents the range of characters allowed in a
username or domainname.  It really states which chars aren't allowed.*/
                                                                                                                                             
var validChars="\[^\\s" + specialChars + "\]";
                                                                                                                                             
/* The following pattern applies if the "user" is a quoted string (in
which case, there are no rules about which characters are allowed
and which aren't; anything goes).  E.g. "jiminy cricket"@disney.com
is a legal e-mail address. */
                                                                                                                                             
var quotedUser="(\"[^\"]*\")";
                                                                                                                                             
/* The following pattern applies for domains that are IP addresses,
rather than symbolic names.  E.g. joe@[123.124.233.4] is a legal
e-mail address. NOTE: The square brackets are required. */
                                                                                                                                             
var ipDomainPat=/^\[(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})\]$/;
                                                                                                                                             
/* The following string represents an atom (basically a series of non-special characters.) */
                                                                                                                                             
var atom=validChars + '+';
                                                                                                                                             
/* The following string represents one word in the typical username.
For example, in john.doe@somewhere.com, john and doe are words.
Basically, a word is either an atom or quoted string. */
                                                                                                                                             
var word="(" + atom + "|" + quotedUser + ")";
                                                                                                                                             
// The following pattern describes the structure of the user
                                                                                                                                             
var userPat=new RegExp("^" + word + "(\\." + word + ")*$");
                                                                                                                                             
/* The following pattern describes the structure of a normal symbolic
domain, as opposed to ipDomainPat, shown above. */
                                                                                                                                             
var domainPat=new RegExp("^" + atom + "(\\." + atom +")*$");
                                                                                                                                             
/* Finally, let's start trying to figure out if the supplied address is valid. */
                                                                                                                                             
/* Begin with the coarse pattern to simply break up user@domain into
different pieces that are easy to analyze. */
            
var matchArray=emailStr.match(emailPat);
                                                                                                                                             
if (matchArray==null) {
                                                                                                                                             
/* Too many/few @'s or something; basically, this address doesn't
even fit the general mould of a valid e-mail address. */
                                                                                                                                             
alert("Dette ser ikke ut som en gyldig epostadresse (sjekk @ og .)");
return false;
}
var user=matchArray[1];
var domain=matchArray[2];
                                                                                                                                             
// Start by checking that only basic ASCII characters are in the strings (0-127).
                                                                                                                                             
for (i=0; i<user.length; i++) {
if (user.charCodeAt(i)>127) {
alert("Brukernavnet inneholder ulovlige tegn.");
return false;
   }
}
for (i=0; i<domain.length; i++) {
if (domain.charCodeAt(i)>127) {
alert("Domenenavnet inneholder ulovlige tegn.");
return false;
   }
}
                                                                                                                                             
// See if "user" is valid
                                                                                                                                             
if (user.match(userPat)==null) {
                                                                                                                                             
// user is not valid
                                                                                                                                             
alert("Brukernavnet ser ut ikke ut til å være gyldig.");
return false;
}
                                                                                                                                             
/* if the e-mail address is at an IP address (as opposed to a symbolic
host name) make sure the IP address is valid. */
                                                                                                                                             
var IPArray=domain.match(ipDomainPat);
if (IPArray!=null) {
                                                                                                                                             
// this is an IP address
                                                                                                                                             
for (var i=1;i<=4;i++) {
if (IPArray[i]>255) {
alert("IP addressen er ugyldig!");
return false;
   }
}
return true;
}
                                                                                                                                             
// Domain is symbolic name.  Check if it's valid.
                                                                                                                                             
var atomPat=new RegExp("^" + atom + "$");
var domArr=domain.split(".");
var len=domArr.length;
for (i=0;i<len;i++) {
if (domArr[i].search(atomPat)==-1) {
alert("Domenenavnet ser ugyldig ut.");
return false;
   }
}
                                                                                                                                             
/* domain name seems valid, but now make sure that it ends in a
known top-level domain (like com, edu, gov) or a two-letter word,
representing country (uk, nl), and that there's a hostname preceding
the domain or country. */
                                                                                                                                             
if (checkTLD && domArr[domArr.length-1].length!=2 &&
domArr[domArr.length-1].search(knownDomsPat)==-1) {
alert("Adressen må slutte med et velkjent domene.");
return false;
}
                                                                                                                                             
// Make sure there's a host name preceding the domain.
                                                                                                                                             
if (len<2) {
alert("Adressen mangler hostnavn!");
return false;
}
                                                                                                                                             
// If we've gotten this far, everything's valid!
return true;
}