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 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("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 username 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 of email address is invalid!")
                   return false
               }
       }
       return true
   }
   
   // Domain is symbolic name
   var domainArray=domain.match(domainPat)
   if (domainArray==null) {
           alert("The domain name of the email address 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), and that there's a hostname preceding 
      the domain or country. */
   
   /* 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
   }
   
   // Make sure there's a host name preceding the domain.
   if (len<2) {
      var errStr="This email address is missing a hostname!"
      alert(errStr)
      return false
   }
   
   // If we've gotten this far, everything's valid!
   return true;
 }