Script question

[Robert Baer]

I have the following code that works to a point:
<script>
// discover if browser is crappy Opera or crappy Safari
str=navigator.userAgent; // Agent is unique for our purposes
var sf=str.indexOf("Safari"); // found if browser is Chrome or Safari
var ch=str.indexOf("Chrome"); // not found if browser is Safari
var op=str.indexOf("Opera"); // unique for Opera
var opt=1; // alternate shown below but not used
ovar=window.opera; // is undefined if browser is not Opera
if (!ovar) { opt=-1; } // unique for Opera
// Safari gives sf=91,ch=-1; Opera gives sf=-1,op=7; Chrome gives
sf=-1,ch>0.
// Jump to different markup if crappy browser
if (sf>0 && ch<0 || op>=0) { window.location.replace('Mobile.html'); }
// ** NOTE treat crap as if mobile phone **
// Code is a one-pager; there are mobile phones that struggle and die with
// message "Page too large" "open media site"

var isIE=str.indexOf("IE"); // not found in other browsers
if (isIE>0) { window.location.replace('PCbase.html'); }
var isMozilla=str.indexOf("Mozilla"); // Presumes in all variants;
includes Netscape
if (isMozilla>=0) { window.location.replace('PCbase.html'); }
if (ch>=0) { window.location.replace('PCbase.html'); } // for Chrome

document.write(str+"<BR>");
document.write("At this point, browser is not Safari, Chrome, Opera or
IE or Mozilla"+"<BR>");
// So, ASSuME we have mobile phone browser
// window.location.replace('Mobile.html');
// -----^ always executes; previous valid .replace somehow bypassed
</script>

If i un-comment that last window.location.replace line, the proper
execution of going to PCbase.html fails, and i get Mobile.html instead.

How can this be fixed?