Apex ID 15 to 18

ApexID15to18.cls

public static string generate18CharId(final string id){
  final String abecedair = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ012345';
  if (String.isBlank(id) || (id.length() != 15 && id.length() != 18)) { 
    return null; 
  } 
  if (id.length() == 18) { 
    return id; 
  } 
  string suffix = '';
  for (integer i = 0; i < 3; i++) { 
    integer flags = 0; 
    for (integer j = 0; j < 5; j++) { 
      string c = id.substring(i * 5 + j,i * 5 + j + 1);
      if (c.isAllUpperCase() && c >= 'A' && c <= 'Z') { 
        flags += (1 << j); 
      } 
    } 
    suffix += abecedair.substring(flags,flags+1); 
  } 
  return id + suffix; 
}