Examples with their respective Code

<SCRIPT LANGUAGE="JavaScript">
function checkField(fld) {
     if (fld.value <= 5) {
          alert("Please enter a number greater than 5");
     }
}
</SCRIPT>

</HEAD>
<BODY>

<FORM>
     <INPUT TYPE="text" onChange="checkField(this)">
</FORM>

<FORM>
     <INPUT TYPE="text"
       onChange="if (this.value <= 5) {
          alert('Please enter a number greater than 5');
       }">
</FORM>