how does this regular expression works

["who.cat" <win.acc@xxxxxxxxx>]

Reading some code from the web spider,got the such a expression :
preg_match("@<head[^>]*>(.*?)<\/head>@si",$file, $regs);
It's about get the head content of a website,i wanna know the match detail
.Could someone give some tips ,
thanks in advance .

All you best
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