function checkForm(theForm, errorTarget, errorText, errorColor) {

  // set global variables
  var formOK = true;

  // loop through each form element
  for(i=0; i<theForm.elements.length; i++) {

    // set item variable
    var theItem = theForm.elements[i];
    var thePattern = "";
    var theLabel = "";

    // determine what item must be validated as
    if(theItem.className.indexOf('valid-content') >= 0) { // content required
      thePattern = /^(.+[\r\n]*)+$/;
    }
    else if(theItem.className.indexOf('valid-letters') >= 0) { // letters only
      thePattern = /^[a-zA-Z]+$/;
    }
    else if(theItem.className.indexOf('valid-numbers') >= 0) { // numbers only
      thePattern = /^[0-9]+$/;
    }
    else if(theItem.className.indexOf('valid-email') >= 0) { // email address format
      thePattern = /^[a-zA-Z0-9\_\-\.]+\@[a-zA-Z0-9\-\.]+\.[a-zA-Z]{2,4}$/;}

    // get item's label
    formLabels = theForm.getElementsByTagName('label');
    for(x=0; x<formLabels.length; x++) {
      if(formLabels[x].htmlFor == theItem.id) {
        theLabel = formLabels[x];
      }
    }

    // if the item was required for validation and failed to validate
    if(thePattern != '' && !thePattern.test(theItem.value)) {
      formOK = false;

      // change field background
      theItem.style.backgroundColor = errorColor;

      // change label color
      theLabel.style.color = errorColor;

      // set change background on focus
      theBG = document.createAttribute('onfocus');
      theBG.nodeValue = "this.style.backgroundColor = ''";
      theItem.setAttributeNode(theBG);
    }
    // if field validates
    else if(thePattern != '' && thePattern.test(theItem.value)) {
      // set background back to default
      theItem.style.backgroundColor = '';

      // set label color back to default
      theLabel.style.color = '';
    }

    // if the element was a select and a valid option was not selected
    if(theItem.className.indexOf('valid-selection') >= 0 && theItem.options[theItem.selectedIndex].value == '') {
      formOK = false;

      // change label color
      theLabel.style.color = errorColor;
    }
    else if(theItem.className.indexOf('valid-selection') >= 0 && theItem.options[theItem.selectedIndex].value != '') {
      // set label color back to default
      theLabel.style.color = '';
    }

    // if the element was a checkbox and is not checked
    if(theItem.className.indexOf('valid-checked') >= 0 && theItem.checked == false) {
      formOK = false;

      // change label color
      theLabel.style.color = errorColor;
    }
    else if(theItem.className.indexOf('valid-checked') >= 0 && theItem.checked == true) {
      // set label color back to default
      theLabel.style.color = '';
    }
  }

  // display error message
  if(!formOK && errorTarget != '') {
    theError = document.getElementById(errorTarget);
    theError.innerHTML = errorText;
    theError.style.color = errorColor;
    theError.style.display = "block";
  }

  // return final value
  return formOK;
}