I wish to retain information on a person's grades which is observed in the variable below. The text that i would like to retain in a clean format is as follows:
  1. Grade: A .....
  2. Single Letters that are followed by a + or - need to be applied a prefix of "Grade: "
  3. Letter preceded by 1 or 2 digits such as 14 As or 3 Bs also need to be applied a prefix of "Grade: "
What I want to discard:
  1. Observation having 1, 2 or 3 digits only
  2. Sentences
I hope this message is drafted clearly. If not then feel free to ask for more clarification



Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input str31 grade_1
"14 As, 7 Bs"                    
"c"                              
"I continue  study in BA  part 2"
"B"                              
"Grade:D"                        
"200BSc"                        
"Grade:B"                        
"888"                            
"A"                              
"67"                            
"As"                            
"B Grade"                        
"3bs"                            
"B"                              
"B+"                            
"A"                              
"B"                              
"B"                              
"Grade B"                        
"5As,3Bs"                        
"73"                            
"2nd"                            
"B"                              
"B"                              
"Grade:A"                        
"Grade B+"                      
"b"                              
"D"                              
"As"                            
"Pass"                          
"B"                              
"5As"                            
"B"                              
"Master continue"                
"B"                              
"Grade: B"                      
"D"                              
"B+"                            
"b"                              
"A+"                            
"888"                            
"5As"                            
"5As"                            
"As"                            
"A"                              
"b"                              
"B"                              
"I am studying"                  
"2.67"                          
"497"                            
"B Grade"                        
"697"                            
"Grade A"                        
"B"                              
"All B's"                        
"distinction"                    
"B Grade"                        
"A"                              
"B"                              
"B Grades"                      
"Grade C"                        
"A"                              
"c"                              
"B"                              
"3Bs"                            
"5 As"                          
"b"                              
"C+"                            
"A"                              
"B"                              
"D"                              
"B"                              
"A"                              
"1100/627, Grade: C"            
"B"                              
"none"                          
"A"                              
"B"                              
"B"                              
"B"                              
"B"                              
"A"                              
"C"                              
"B"                              
"b"                              
"Grade B"                        
"C"                              
"B"                              
"C"                              
"4"                              
"b"                              
end