Range comparison for alphanumeric

Question

There is a Branch DDL(list ATL1,ATL2) and 2 user input textboxes starteLetterGroupFrom and EndLetterGroupThru.

If the branch selected is 'ATL1' then the above 2 textboxes should have values only between of 000000 - JZZZZZ. For 'ATL2' it should have values between K00000 - ZZZZZZ.

Ex: if user tries to select ATL2 and enters AAAAAA-BZZZZZ in textboxes then it should not allow. similarly,ATL1 cannot be assigned to SAAAAA-ZZZZZZ

    var branch = document.getElementById("selectBranch");
    var branchText = branch.options[branch.selectedIndex].text
    var startltr = $('#inputStartLetter').val().toUpperCase()
    var endltr = $('#inputEndLetter').val().toUpperCase();