Validating a File Name with JS

window.onload = initForms;

function initForms() {
     for (var i=0; i< document.forms.length; i++) {
        document.forms[i].onsubmit = function() {return validForm();}
     }
}

function validForm() {
     var allGood = true;
     var allTags = document.getElementsByTagName ("*");

     for (var i=0; i<allTags.length; i++) {
        if (!validTag(allTags[i])) {
           allGood = false;
        }
     }
     return false;

     function validTag(thisTag) {
        var outClass = "";
        var allClasses = thisTag.className.split (" ");

        for (var j=0; j<allClasses.length; j++) {
           outClass += validBasedOnClass(allClasses[j]) + " ";
        }

        thisTag.className = outClass;

        if (outClass.indexOf("invalid") > -1) {
           invalidLabel(thisTag.parentNode);
           thisTag.focus();
           if (thisTag.nodeName == "INPUT") {
              thisTag.select();
           }
           return false;
           }
           return true;

           function validBasedOnClass(thisClass) {
              var classBack = "";
              switch(thisClass) {
                 case "":
                 case "invalid":
                    break;
                 case "imgURL":
                    if (allGood && !imgURL (thisTag.value)) classBack = "invalid ";
                 default:
                    classBack += thisClass;
              }
              return classBack;
           }

           function imgURL(newURL) {
              var re = /^(file|http):\/\/\S+  \/\S+\.(gif|jpg)$/i;

              if (re.test(newURL)) {
                 document.getElementById ("chgImg").src = newURL;
                 return true;
              }
              return false;
           }

           function invalidLabel(parentTag) {
              if (parentTag.nodeName == "LABEL") {
                 parentTag.className += " invalid";
              }
          }
      }
}