Simple regular expression substitution related issues

Simple Regular expression substitution problem
See a topic from somewhere else:

In the HTML language, the meta tag of the header of the page can be used to output the encoded format of the file, and the following is a standard META statement

Use the PHP language to write a function that changes the value of the charset part of a standard HTML page like a meta tag to Big5
Please note:
1. The full HTML page needs to be processed, i.e. not only this meta statement
2. Ignore case
3. ' and ' can be interchanged in this place
4. The quotation marks on both sides of ' content-type ' are negligible, but ' text/html; CHARSET=GBK ' on both sides of the No
5. Pay attention to handling extra spaces

The first thought is the regular expression substitution, as follows:

PHP Code

  
   $content = "AB
  
    B";//
  
    //preg_match ('/]+) charset=[a-za-z-0-9]+ in the simulated HTML code
  
   
    
   ([\ ' "]) >/i ', $content, $matches)//var_dump ($matches);//This statement can be matched to preg_replace ('/
   
    
     
    ]+) charset=[ a-za-z-0-9]+ ([\ ' "]) >/i ',"
    
     
      
     ", $content); Echo $content;  Output ABB, that is, the replacement is unsuccessful, replace the matched
    part with the empty
     
   
    
  
   

The above is replaced, where there is a problem, seek guidance!

------Solution--------------------
No problem

PHP code

 $content = Preg_replace ('/
   ]+) charset=[a- za-z-0-9]+ ([\ ' "]) >/i '," 
   
     ", $content), Echo ' 
    
 '; e Cho $content; 
 