How does regular matching-php regular expressions match?

Regular match PHP regular expression

[0] = =
   
    
     1
     
      
        2012-2013 school year first semester 
       
 [1] = 
        
        
          2 
          
          
 
          
            [P209502] Management Fundamentals 
           
 [2] = 
            
             
             
               2012-2013 second semester of academic year 
              
 [3] = 
               
                
                 
                 
 
                 
                   [P209500] Fundamentals of Economics 
                  
  
                  
                 
               
                      
             
            
           
          
         
        
       
      
     
    
   

As I did with the regular match.
Preg_match_all ('/(\d) *<\/td> (\s) +?
<\/td>/',
But I just need to:

  
  
    
   
     
      
    1
    
      
       
     2012-2013学年第一学期
     
       
     
     
       
      
        
          13 
         
           2012-2013学年第二学期 
          
 
          
        
     
       
    
      
   
     
  
    

How do I write?

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