r - Regex and proper use of not -

I have the following vectors of variable names:

  vn & lt; - c ("id", "pid", "wave", "px", "pdobd", "pidob", "pdb", "psat3", "ppa 211", "psd23i19")   
 I'm having trouble coming up with a proper regex which removes all the first p, but not for "pid".  
  sub (???, "", vn) ## I tried "^ p [^ id]?" But there is no benefit ...   
 Then, the result will look like this:  
  c ("id", "pid", "wave", "    
 
 You must use (negative lookhead)  
  ^ p (?! Id)   
 TRUE perl parameter