Identify OS from user agent string

I'm trying to come up with a quick and dirty way to identify OS by regexing the user agent string. I'm only interested in Windows (with version), Mac, or Linux - anything else is unknown. In anyone can sanity check my regex or offer any pointers on parsing user agent strings that would be much appreciated. thanks in advance.

PHP Code:
if (eregi("(win.*NT 5.1|win.*NT 5.1|Windows XP)", $agent)) 
{ 
    $visos = "Windows XP"; 
} 
elseif (eregi("(win.*NT 5.0)", $agent)) 
{ 
    $visos = "Windows 2000"; 
} 
elseif (eregi("(win.*NT 5.2)", $agent)) 
{ 
    $visos = "Windows Server"; 
} 
elseif (eregi("win.*NT)", $agent)) 
{ 
    $visos = "Windows NT"; 
} 
elseif (eregi("(win.*98)", $agent)) 
{ 
    $visos = "Windows 98"; 
} 
elseif (eregi("(win 9x.*4.90)", $agent)) 
{ 
    $visos="Windows ME"; 
} 
elseif (eregi("win.*95)", $agent)) 
{ 
    $visos = "Windows 95"; 
} 
elseif ((ereg("(Mac|PPC|Mac_PowerPC)", $agent)) 
{ 
    $visos = "Mac"; 
} 
elseif (eregi("Linux", $agent)) 
{ 
    $visos = "Linux"; 
}   
else 
{ 
    $visos = "unknown"; 
} 

 

 

 

 

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