Javascript function to determine value in textbox

Question

This should have been a realtively easy function to write.. but it seems as if there should be a quicker way to do part of it... can anyone give me a hand.
I have a text box the the user enters a code representing the page they want to go to. Depending on the page they want I set a variable. The code can be numbers or letters up to 6 in length. If the code comes in as XXXXDT then I do one thing.. this is done. But if the code comes in as TXXX I have to do something. I can check for the "T" but to check to make sure the other 3 digits are numeric I'm not sure what the best way would be. Can I just take the substring and do a parseInt on it... but if someone where to enter "TTTT" then the parseInt would crash... and I've never done any error handling in javascript.
this is the code I have so far
function goToNextScreen(frm){
              var url = window.location + "";     
              var pathPrepend ="";
              var newAction="&omniActionOrTab=Refresh";
              var screen = document.forms["GoToScreen"].screenName.value.toUpperCase();
              if(url.indexOf("/jsp") < 0)
                pathPrepend += "jsp/";     
               if(screen.length==6&&screen.substring(4)=="DT"){
                    newAction="&omniActionOrTab=Init";
               }else if(screen.length==4&&screen.substring(0,1)=="T"){
                               //do more here to check to make sure the next three characters
                              // are numbers.
               document.forms["GoToScreen"].action = pathPrepend + document.forms["GoToScreen"].screenName.value.toUpperCase() + ".do?useSessionKeys=true" + newAction;               
               alert("action = "+document.forms["GoToScreen"].action);
               return true;
          }          

Answer 1

I have never used regular expressions. So after reading the page you sent me this is what I came up with... but when I enter a value of "T444" the if statement is not being stepped into... could you tell me what I'm doing wrong?
function goToNextScreen(frm){
              var url = window.location + "";     
              var pathPrepend ="";
              var newAction="&omniActionOrTab=Refresh";
              var screen = document.forms["GoToScreen"].screenName.value.toUpperCase();
              tranRegexp = new RegExp("/T(\d){3}/");                 
              detailRegexp = new RegExp("/(\w){4}DT/");         
              if(url.indexOf("/jsp") < 0)
                pathPrepend += "jsp/";      
                alert("tranRegexp.exec(screen)="+tranRegexp.exec(screen));
                alert("detailRegexp.exec(screen)="+detailRegexp.exec(screen));
               if(detailRegexp.exec(screen) != null){
                    alert("inside1");
                    newAction="&omniActionOrTab=Init";
               }else if(tranRegexp.exec(screen) != null){
                    alert("inside2");
                    newAction="omniActionOrTab=Init";
                  alert("inside3");
               document.forms["GoToScreen"].action = pathPrepend + document.forms["GoToScreen"].screenName.value.toUpperCase() + ".do?useSessionKeys=true" + newAction;               
               alert("action = "+document.forms["GoToScreen"].action);
               return true;
          }