Form Validation for 24 hour format

Hi All,

I've tried to get a form validation working for 24 hour format but it just doesn't work, what am I doing wrong???  It needs to allow such an example as 02:30 or 22:45, etc.

Here's my code (it's the if statement that checks the field StartTime that is the issue)

function CheckFormEditTaskHeader(FormTaskHeader){
    if (document.FormTaskHeader.JobName.value == "")
    {
            alert("You need to enter a job name.  \r\rPlease try again.")
            document.FormTaskHeader.JobName.focus();                  
            return false;
      }
      if (document.FormTaskHeader.ManagerID.value == "")
    {
            alert("You need to enter a manager.  \r\rPlease try again.")
            document.FormTaskHeader.ManagerID.focus();                  
            return false;
      }
      if (document.FormTaskHeader.JobType.value == "")
    {
            alert("You need to enter a job type.  \r\rPlease try again.")
            document.FormTaskHeader.JobType.focus();                  
            return false;
      }
         if (document.FormTaskHeader.StartTime.value != "") {
            var rg = new RegExp(/^([01][0-9]|2[0-3]):[0-5][0-9]$/);
            str.match(rg)
            if (str.match(rg) == null) {
            alert("You need to make sure the first letter is a J or j followed by atleast 5 numbers, for example J50001. \r\rPlease try again.")
            document.FormTaskHeader.StartTime.focus();                  
            return false;
            }
      return true;
      }            
    return true;
}


Thanks in advance for any help.
Neilos