Need to alter a javascript function that looks at a specific character in a string, look for acceptable range of values.

Hello,

I have a javascript function that looks at a string value passed to it.  It is currently setup to look at the first 2 characters in the string, and if they are equal to a certain string, then do something, if not, do something else.  Anyway, I need to change that function to look at the first character and the second character independantly.  Kinda confusing, so here is the current function:

function bar_unit(f){
  if (Left(f.unit_num.value, 2)=="=W"){
        var unit_num = Right(f.unit_num.value, 15);
      var unit_num = Left(unit_num, 13);
       f.unit_num.value = unit_num;
  }
  else{
        return;
  }
}

Now what I want to do is check to be sure the first character is "=", then check the 2nd character and see if it is either A-Z or 1-9.  If the first charcter is "=" and the second is "A-Z" OR "1-9", then do the var unit_num part.  If either of those conditions are false (being the first "=" or the second is not A-Z or 1-9",  then return;  Thanks!
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