function emailCheck (emailStr) {

/* 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 represents the range of characters allowed as
   the first character in a valid username or domain.  I just made it
   the same as above, but if you want to add a different constraint,
   you would change it here. */

var firstChars=validChars

/* 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 at atom (basically a series of
   non-special characters.) */

var atom="(" + firstChars + 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 course 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("Email address seems incorrect (check @ and .'s)")
	return false
}
var user=matchArray[1]
var domain=matchArray[2]

// See if "user" is valid 
if (user.match(userPat)==null) {
    // user is not valid
    alert("The Email Address doesn't seem to be valid.")
    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("Destination IP address is invalid!")
		return false
	    }
    }
    return true
}

// Domain is symbolic name
var domainArray=domain.match(domainPat)
if (domainArray==null) {
	alert("The domain name doesn't seem to be valid.")
    return false
}
/* domain name seems valid, but now make sure that it ends in a
   three-letter word (like com, edu, gov) or a two-letter word,
   representing country (uk, nl).
   If there's a country code at the end of the address, the full domain
   must include a hostname and category (e.g. host.co.uk or host.pub.nl).
   If it ends in a .com or something, make sure there's a hostname.*/

/* Now we need to break up the domain to get a count of how many atoms
   it consists of. */
var atomPat=new RegExp(atom,"g")
var domArr=domain.match(atomPat)
var len=domArr.length
if (domArr[domArr.length-1].length<2 || 
    domArr[domArr.length-1].length>3) {
   // the address must end in a two letter or three letter word.
   alert("The Email address must end in a three-letter domain, or two letter country.")
   return false
}

/* If it ends in a country code, we want to make sure there are at
   least 2 atoms preceding it (representing host and category (i.e.
   com, gov, etc.)) */
if (domArr[domArr.length-1].length==2 && len<3) {
   var errStr="This address ends in two characters, which is a country"
   errStr+=" code.  Country codes must be preceded by "
   errStr+="a hostname and category (like com, co, pub, pu, etc.)"
   alert(errStr)
   return false
}

/* If it just ends in .com, .gov, etc., make sure there's a host name.
   This case can never actually happen because earlier checks take
   care of this implicitly, but we'll do it anyway. */
if (domArr[domArr.length-1].length==3 && len<2) {
   var errStr="This address is missing a hostname!"
   alert(errStr)
   return false
}
// If we've gotten this far, everything's valid!
return true;
}