/** utility functions for formdata validation */
function checkText(txtField, req, message) {
  var s = txtField.value;
  if(s == "") {
    if(req) {
      alert(message);
      txtField.focus();
      txtField.select();
      return false;
    }
  }
  return true;
}

function checkSelect(txtField, req, message) {
  var s = txtField.value;
  if(s == "") {
    if(req) {
      alert(message);
      txtField.focus();
      return false;
    }
  }
  return true;
}

function checkNumber(txtField, req, message) {
  var s = txtField.value;
  if(s == "") {
    if(req) {
      alert(message + " must be entered.");
      txtField.focus();
      txtField.select();
      return false;
    }
  }
  else {
    if(isNaN(s)) {
      alert(message + " must be numeric.");
      txtField.focus();
      txtField.select();
      return false;
    }
  }
  return true;
}

function checkLength(txtField, rLength, message) {
  var s = txtField.value;
  if(s.length != rLength) {
     alert(message + " must be " + rLength + " digits.");
     txtField.focus();
     txtField.select();
     return false;
  }
  return true;
}

function checkRange(txtField, min, max, message) {
  var s = txtField.value;
  if((s < min) || (s > max)) {
     alert(message + " must be between " + min + " and " + max + ".");
     txtField.focus();
     txtField.select();
     return false;
  }
  return true;
}

function checkTermLength(txtFieldTerm, txtFieldLength, rLength, message) {
  var s1 = txtFieldTerm.value;
  var s2 = txtFieldLength.value;
  if(s1.toLowerCase() == "annual" && s2 < 12) {
     alert(message + " must be " + rLength + " months.");
     txtFieldLength.focus();
     txtFieldLength.select();
     return false;
  }
  return true;
}

function checkYear(txtField, req, message) {
  var yyyy = txtField.value;
  if(yyyy == "") {
    if(req) {
      alert("Please enter year for " + message + ".");
      txtField.focus();
      txtField.select();
      return false;
    }
  }
  else {
    if(!(isNaN(yyyy))) {
      //if((yyyy >= 1000) && (yyyy <= 9999)) {
        return true;
      //}
    }
    alert("Invalid year for " + message + ".");
    txtField.focus();
    txtField.select();
    return false;
  }
  return true;
}

function checkMonth(txtField, req, message) {
  var mm = txtField.value;
  if(mm == "") {
    if(req) {
      alert("Please enter month for " + message + ".");
      txtField.focus();
      txtField.select();
      return false;
    }
  }
  else {
    if(!(isNaN(mm))) {
      if((mm >= 1) && (mm <= 12)) {
        return true;
      }
    }
    alert("Invalid month for " + message + ".");
    txtField.focus();
    txtField.select();
    return false;
  }
  return true;
}

function checkDay(txtField, req, message, txtMonth) {
  var dd = txtField.value;
  var mm = txtMonth.value;
  if(dd == "") {
    if(req) {
      alert("Please enter day for " + message + ".");
      txtField.focus();
      txtField.select();
      return false;
    }
  }
  else {
    if(!(isNaN(dd))) {
      var numDays = new Array(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
      if((dd > 0) && (dd <= numDays[mm - 1])) {
        return true;
      }
    }
    alert("Invalid day for " + message + ".");
    txtField.focus();
    txtField.select();
    return false;
  }
  return true;
}



/**********************************************************************
Form validation functions involving REGEX 
**********************************************************************/
//If string is empty
function isEmpty(frmElement, message) {
  if (frmElement.value.length == 0) {
    alert(message);
    frmElement.focus(); // set the focus to this input
    frmElement.select();
    return true;
  }
  return false;
}


//If string matches the regular expression it is all numbers [0-9]
function isNumeric(frmElement, message) {
  var numExp = /^[0-9]+$/;
  if (frmElement.value.match(numExp)) {
    return true;
  }
  else {
    alert(message);
    frmElement.focus();
    frmElement.select();
    return false;
  }
}


//If string matches the regular expression it is all letters [a-zA-Z]
function isAlphabet(frmElement, message) {
  var alphaExp = /^[a-zA-Z]+$/;
  if (frmElement.value.match(alphaExp)) {
    return true;
  }
  else {
    alert(message);
    frmElement.focus();
    frmElement.select();
    return false;
  }
}


//If string matches the regular expression it is numbers [0-9] and letters [a-zA-Z] only
function isAlphanumeric(frmElement, message) {
  var alphanumExp = /^[0-9a-zA-Z]+$/;
  if (frmElement.value.match(alphanumExp)) {
    return true;
  }
  else {
    alert(message);
    frmElement.focus();
    frmElement.select();
    return false;
  }
}


//If string length is within the range [min, max] specified
function isLengthRange(frmElement, minLength, maxLength, message) {
  var strInput = frmElement.value;
  if (strInput.length >= minLength && strInput.length <= maxLength){
    return true;
  } 
  else {
    alert(message);
    frmElement.focus();
    frmElement.select();
    return false;
  }
}


function isEmail(frmElement, message) {
  var emailExp = /^[\w\-\.\+]+\@[a-zA-Z0-9\.\-]+\.[a-zA-z0-9]{2,4}$/;
  if (frmElement.value.match(emailExp)) {
    return true;
  } 
  else {
    alert(message);
    frmElement.focus();
    frmElement.select();
    return false;
  }
}



/*****************************************************
 * Functions used for validating credit cards
******************************************************/
  /** public boolean isCardNumber(String cardType, String cardNumber) { */
  function isCardNumber(cardType, cardNumber) {
    //if (! isNum(cardNumber)) {
    //  return false;
    //}

    if (! luhnCheck(cardNumber) ) {
      return false;
    }

    if (! matchCardRules(cardType, cardNumber) ){
      return false;
    }

    return true;
  }

  /** public boolean matchCardRules(String cardType, String cardNumber) { */
  function matchCardRules(cardType, cardNumber) {
    var cardPrefixes = getCardPrefixes(cardType);
    var cardLengths = getCardLengths(cardType);

    //if (cardPrefixes.size == 2) return true;
    //if (cardLengths.size == 1) return true;

    for (var i = 0; i < cardLengths.length; i++) {
      if (cardNumber.length == cardLengths[i]) {
        for (var j = 0; j < cardPrefixes.length; j++) {
          if (cardNumber.substring(0, cardPrefixes[j].length) == cardPrefixes[j]) {
            return true;
          }
        }
        return false;
      }
    }

    return false;
  }

  /** public Object getCardPrefixes(String cardType) { */
  function getCardPrefixes(cardType) {
    var visaPrefixes = ["4"];
    var masterCardPrefixes = ["51", "52", "53", "54", "55"];
    var discoverPrefixes = ["6011"];
    var amexPrefixes = ["34", "37"];
    var defaultPrefixes = ["4", "51", "52", "53", "54", "55", "6011", "34", "37"];
    
    if (cardType.toLowerCase() == "visa") {
      return visaPrefixes;
    }
    if (cardType.toLowerCase() == "mastercard") {
      return masterCardPrefixes;
    }
    if (cardType.toLowerCase() == "discover") {
      return discoverPrefixes;
    }
    if (cardType.toLowerCase() == "amex" || cardType.toLowerCase() == "americanexpress") {
      return amexPrefixes;
    }

    return defaultPrefixes;
  } 

  /** public Object getCardLengths(String cardType) { */
  function getCardLengths(cardType) {
    var visaLengths = ["13", "16"];
    var masterCardLengths = ["16"];
    var discoverLengths = ["16"];
    var amexLengths = ["15"];
    var defaultLengths = ["13", "15", "16"];
    
    if (cardType.toLowerCase() == "visa") {
      return visaLengths;
    }
    if (cardType.toLowerCase() == "mastercard") {
      return masterCardLengths;
    }
    if (cardType.toLowerCase() == "discover") {
      return discoverLengths;
    }
    if (cardType.toLowerCase() == "amex" || cardType.toLowerCase() == "americanexpress") {
      return amexLengths;
    }

    return defaultLengths;
  } 

  /** public boolean isExpireDate(String year, String month) { */
  function isExpireDate(year, month) {
    if (!isNum(year) || !isNum(month)) {
      return false;
    }

    if (year.length != 4 || month.length != 2) {
      return false;
    }

    var today = new Date(); //get current date and time
    //var expDate = new Date(parseInt(year), parseInt(month));
    var expDate = new Date(year, month);

    if (expDate.getTime() < today.getTime()) {
      return false;
    }

    return true;
  }

  /** public boolean isNum(String number) { */
  function isNum(number) {
    if (number.length == 0) {
      return false;
    }

    for (var i = 0; i < number.length; i++) {
      if (number.charAt(i) < "0" || number.charAt(i) > "9") {
        return false;
      }
    }

    return true;
  }

  /** public boolean luhnCheck(String cardNumber) { */
  function luhnCheck(cardNumber) {
    if (! isNum(cardNumber)) {
      return false;
    }

    var tmpCardNumber;
    var digit = 0;
    var sum = 0;

    /** If cardNumber contains odd number of digits, then convert cardNumber
     *  to even number of digits by prefixing it with a "0" (which is harmless
     *  when used for the purpose of LuhnCheck).
     */
    if (cardNumber.length % 2 == 0) {
      tmpCardNumber = cardNumber;
    }
    else {
      tmpCardNumber = "0" + cardNumber;
    }

    for (var i = 0; i < tmpCardNumber.length; i++) {
      digit = parseInt(tmpCardNumber.charAt(i));
      if (i % 2 == 0) {
        digit = digit * 2;  //digit *= 2;
        if (digit > 9) {
          digit = digit - 9;  //digit -= 9;
        }
      }
      sum = sum + digit;  //sum += digit
    }
    if (sum % 10 == 0) {
      return true;
    }
    return false;
  }



/*****************************************************************************************************
References for form validation involving REGEX 
[^a-zA-Z]
[^0-9]


//If the element's string matches the regular expression it is all numbers
function isNumeric(elem, helperMsg){
	var numericExpression = /^[0-9]+$/;
	if(elem.value.match(numericExpression)){
		return true;
	}else{
		alert(helperMsg);
		elem.focus();
		return false;
	}
}


// If the element's string matches the regular expression it is all letters
function isAlphabet(elem, helperMsg){
	var alphaExp = /^[a-zA-Z]+$/;
	if(elem.value.match(alphaExp)){
		return true;
	}else{
		alert(helperMsg);
		elem.focus();
		return false;
	}
}


// If the element's string matches the regular expression it is numbers and letters
function isAlphanumeric(elem, helperMsg){
	var alphaExp = /^[0-9a-zA-Z]+$/;
	if(elem.value.match(alphaExp)){
		return true;
	}else{
		alert(helperMsg);
		elem.focus();
		return false;
	}
}


function lengthRestriction(elem, min, max){
	var uInput = elem.value;
	if(uInput.length >= min && uInput.length <= max){
		return true;
	}else{
		alert("Please enter between " +min+ " and " +max+ " characters");
		elem.focus();
		return false;
	}
}


function emailValidator(elem, helperMsg){
	var emailExp = /^[\w\-\.\+]+\@[a-zA-Z0-9\.\-]+\.[a-zA-z0-9]{2,4}$/;
	if(elem.value.match(emailExp)){
		return true;
	}else{
		alert(helperMsg);
		elem.focus();
		return false;
	}
}


//----------------------------------
Example:

I am trying to construct a reg exp for a field which can accept values from [A-Za-z0-9]. It cannot start 
with a number and cannot have only numbers. Its maximum length is 50. Apart from the above characters it 
can only include the symbols Underscore, Hyphen and spaces between words. 

/^[a-z][-a-z0-9_ ]{0,49}$/i 

Breakdown: 
/^ 
Begin match at beginning of string 

[a-z] 
Match one alpha character 

[-a-z0-9_ ]{0,49} 
Match zero to fourtynine occurences of one of the following: alpha, digit, hyphen, underscore and space 

$ 
Match end of string 

/i 
Append case insensitivity flag (Makes the regex smaller than the /^[A-Za-z][-A-Za-z0-9_ ]{0,49}$/ it 
would have been without case insensitivity flag) 
******************************************************************************************************/