Need to redirect form to multiple URLs based on input

I have a basic HTML form and I need help creating a bit of JS to redirect my form to different URLs based on the string typed in a text field.

<form class="form-inline">
    <div class="form-group">
        <input type="text">
        <button type="submit">Go</button>
    </div>
</form>

There will be 3 or 4 strings of text - to be entered in the input field - that are "valid" and I need them to make the form redirect to various pages on the site.

For instance, typing valid string "STRING1" would make the page redirect to example.com/something.html on form submit, or "STRING2" to example.com/otherpage.html.

But invalid strings would need to go to a page like "example.com/invalid.html."

The most useful thing I've seen so far is this guide: http://www.dynamicdrive.com/forums/showthread.php?20294-Form-POST-redirect-based-on-radio-button-selected

<script type="text/javascript">
function usePage(frm,nm){
    for (var i_tem = 0, bobs=frm.elements; i_tem < bobs.length; i_tem++)
    if(bobs[i_tem].name==nm&&bobs[i_tem].checked)
    frm.action=bobs[i_tem].value;
 }
</script>

In that code, each radio has a value assigned to it. But this doesn't help with text fields or having a blanket redirect if the string is invalid.

Thanks so much for your help.