How does this sound :
function validate(elem)
{
var anum=/(^923(\d){10}$)/; //using this regex after the number 923
// we are allowed to have 10 digits
if(anum.test(elem.value) == true)
{
elem.style.border="2px solid #0F0"; //green light
}
else
{
elem.style.border="2px solid #F00"; //red light
}
}
And in the html we have :
<input type="text" id="phone" onchange="validate(this)" />