function check_email( 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("Your 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("Your email username doesn't seem to be valid (everything left of the @ symbol).")
        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("Your email destination IP address is invalid!")
                return false
            }
        }
        return true
    }
    
    // Domain is symbolic name
    var domainArray=domain.match(domainPat)
    if (domainArray==null) {
        alert("Your email domain name doesn't seem to be valid.")
        return false
    }
    
    /* domain name seems valid, but now make sure that it ends in a four-letter
       word (like info), 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>4) {
       // the address must end in a two letter or three letter word.
       alert("Your email address must end in a three-letter domain, or two letter country code.")
       return false
    }
    
    // Make sure there's a host name preceding the domain.
    if (len<2) {
       alert("Your email address is missing a hostname!");
       return false
    }
    
    // If we've gotten this far, everything's valid!
    return true;
}