function showhide( id, val ) {
  var element = document.getElementById(id);
  if( val == "show" ){
    element.className = "show";
  } else {
    element.className = "hide";
  }
}

//source: http://www.nmitchell.co.uk/code/javascript.htm
function isWhite(c) {
    return ((c == ' ') || (c == ' ') || (c == '\t') || (c == '\n') || (c == '\r'));
}
function trim(s){
    var i, j = s.length;

    for (i = 0; isWhite(s.charAt(i)) && (i < j); i++)
        ;
    if (i == j) return ""; //whole string is white
    for (; isWhite(s.charAt(j-1)); j--)
        ;
    return s.substring(i, j);
}

function ClearErrors() {
  // reset email error messages
  showhide( "no_email_error", "hide" );
  showhide( "email_error", "hide" );
    showhide( "atdot_email_error", "hide" );
    showhide( "userchar_email_error", "hide" );
    showhide( "domainchar_email_error", "hide" );
    showhide( "baduser_email_error", "hide" );
    showhide( "badip_email_error", "hide" );
    showhide( "baddomain_email_error", "hide" );
    showhide( "badhost_email_error", "hide" );
  
  //reset subject and message errors
//  showhide( "subject_error", "hide" );
  showhide( "message_error", "hide" );
}

function SubmitContactForm() {
  // reset error messages
  ClearErrors()
  
  // get field values
  var address = trim( document.contactform.address.value );
  var subject = trim( document.contactform.subject.value );
  var message = trim( document.contactform.message.value );
  
  // check non-essential fields
  if( address == "" ) {
    address = document.contactform.address.value = "Anonymous";
  }
  if( subject == "" ) {
    subject = document.contactform.subject.value = "No Subject";
  }
  
  // ensure message field is filled in
  if( message != "" ) {
    // check if address is "Anonymous"
    if( address.toLowerCase() == "anonymous" ) {
      document.contactform.action = "send.php";
      return true;
    }
    else {
      // check that the address appears to be a valid email address
      var errorID = emailCheck( address );
      if( errorID == "" ) {
        // everything's fine; send their email
        document.contactform.action = "send.php";
        return true;
      }
      else {
        // show appripriate error message for the problem
        showhide( "email_error", "show" );
        showhide( errorID, "show" );
      }
    }
  }
  // mark empty fields and e-mail errors
  else {
    if( address.toLowerCase() != "anonymous" ) {
      var errorID = emailCheck( address );
      if( errorID != "" ) {
        // show appripriate error message for the problem
        showhide( "email_error", "show" );
        showhide( errorID, "show" );
      }
    }
    if( message == "" ) {
      showhide( "message_error", "show" );
    }
  }

  return false;
}

<!-- This script and many more are available free online at -->
<!-- The JavaScript Source!! http://javascript.internet.com -->

<!-- V1.1.3: Sandeep V. Tamhankar (stamhankar@hotmail.com) -->
<!-- Original:  Sandeep V. Tamhankar (stamhankar@hotmail.com) -->
<!-- Changes:
/* 1.1.4: Fixed a bug where upper ASCII characters (i.e. accented letters
international characters) were allowed.

1.1.3: Added the restriction to only accept addresses ending in two
letters (interpreted to be a country code) or one of the known
TLDs (com, net, org, edu, int, mil, gov, arpa), including the
new ones (biz, aero, name, coop, info, pro, museum).  One can
easily update the list (if ICANN adds even more TLDs in the
future) by updating the knownDomsPat variable near the
top of the function.  Also, I added a variable at the top
of the function that determines whether or not TLDs should be
checked at all.  This is good if you are using this function
internally (i.e. intranet site) where hostnames don't have to 
conform to W3C standards and thus internal organization e-mail
addresses don't have to either.
Changed some of the logic so that the function will work properly
with Netscape 6.

1.1.2: Fixed a bug where trailing . in e-mail address was passing
(the bug is actually in the weak regexp engine of the browser; I
simplified the regexps to make it work).

1.1.1: Removed restriction that countries must be preceded by a domain,
so abc@host.uk is now legal.  However, there's still the 
restriction that an address must end in a two or three letter
word.

1.1: Rewrote most of the function to conform more closely to RFC 822.

1.0: Original  */
// -->
<!-- Begin
function emailCheck( emailStr ) {
  // 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 = /^(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.
    return "atdot_email_error";
  }
  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 ) {
      return "userchar_email_error";
    }
  }
  for( i = 0; i < domain.length; ++i ) {
    if( domain.charCodeAt(i) > 127 ) {
      return "domainchar_email_error";
    }
  }
  
  // See if "user" is valid 
  if( user.match( userPat ) == null ) {
    // user is not valid
    return "baduser_email_error";
  }
  
  // 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 ) {
        return "badip_email_error";
      }
    }
    return "";
  }
  
  // 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 ) {
      return "baddomain_email_error";
    }
  }
  
  // Make sure there's a host name preceding the domain.
  if( len < 2 ) {
    return "badhost_email_error";
  }
  
  // If we've gotten this far, everything's valid!
  return "";
}
//  End -->