function isValidEmail(str) {
	var at = "@"
	var dot = "."
	var lat = str.indexOf(at)
	var lstr = str.length
	var ldot = str.indexOf(dot)
	var ret = true;
	if (str.indexOf(at) == -1){
	   ret = false
	} else if (str.indexOf(at)==-1 || str.indexOf(at)==0 || str.indexOf(at)==lstr){
	   ret = false
	} else if (str.indexOf(dot)==-1 || str.indexOf(dot)==0 || str.indexOf(dot)==lstr){
	   ret = false
	} else if (str.indexOf(at,(lat+1))!=-1){
	   ret = false
	} else if (str.substring(lat-1,lat)==dot || str.substring(lat+1,lat+2)==dot){
	   ret = false
	} else if (str.indexOf(dot,(lat+2))==-1){
	   ret = false
	} else if (str.indexOf(" ")!=-1){
	   ret = false
	}
	if (ret == false) {
		alert("Zadali jste Email ve špatném tvaru!")
	}
	return ret					
}

// Declaring required variables
var digits = "0123456789";
// non-digit characters which are allowed in phone numbers
var phoneNumberDelimiters = "()- ";
// characters which are allowed in international phone numbers
// (a leading + is OK)
var validWorldPhoneChars = phoneNumberDelimiters + "+";
// Minimum no of digits in an international phone no.
var minDigitsInIPhoneNumber = 10;

function isInteger(s) {
    var i;
    for (i = 0; i < s.length; i++) {   
        // Check that current character is number.
        var c = s.charAt(i);
        if (c < "0" || c > "9") {
        	return false;
        }
    }
    // All characters are numbers.
    return true;
}

function stripCharsInBag(s, bag) {
    var i;
    var returnString = "";
    // Search through string's characters one by one.
    // If character is not in bag, append to returnString.
    for (i = 0; i < s.length; i++) {   
        // Check that current character isn't whitespace.
        var c = s.charAt(i);
        if (bag.indexOf(c) == -1) returnString += c;
    }
    return returnString;
}

function isValidPhoneNumber(strPhone){
	s = stripCharsInBag(strPhone, validWorldPhoneChars);
	return (isInteger(s) && s.length >= minDigitsInIPhoneNumber);
}

function isNumeric(field) {
	var sText = field.value
   	var ValidChars = "0123456789.";
  	var IsNumber = true;
   	var Char;
 
   	for (i = 0; i < sText.length && IsNumber == true; i++) { 
      	Char = sText.charAt(i); 
      	if (ValidChars.indexOf(Char) == -1) {
         	IsNumber = false;
         	alert("Zadejte prosím číselnou hodnotu.");
         	field.value = ""
      	}
   }
   return IsNumber;
}