Vim regexp example

I have a file : 

alert

  alert

  //alert

So, line 1 has "alert" without any white spaces.

line 2 has white spaces followed by "alert"

line 3 has white spaces,"//" followed by "alert"

I want to find "alert" which is not commented, i.e. not following "//"

Here is the regexp : 

/\(^\|[^\/\/]\)alert  

which is basically : 

(beginning of line OR absense of "//") followed by "alert"