PHP code that intercepts strings and retains complete xml tags

 /**
     * author: goosman
     * blog: http://blog.csdn.net/lgg201
     * mail: lgg860911@yahoo.com.cn
*/

 $str = '01234567890120123456789';
 function substr_remain_tag($s, $o, $l) {
 $is_match = preg_match_all(<< ;
 #This regular expression parses xml tags. The tag attribute internally supports the escape character "", and supports the escaping of "" itself and the corresponding quotation marks
 <( w+) #Start of tag
 (?: #Attribute list
 s+ #Preceding space
 w+ #Attribute name
 s* #Blank after attribute name (for compatibility)
 = #Equal sign between attribute name values
 s* #Blank before attribute value (for compatibility)
 (?: #Attribute value (quote processing)
 " #Double quote situation
 (?:
 \\\\ #Eat two consecutive escape characters (indicates escape symbol itself)
 |
 \\" #Eat the escape character followed by a quote (representing an escaped quote)
 |
 [^"\\]* #Other characters
 )*
 "
 |
 ' #Single quote Case
 (?:
 \\\\ #Eat two consecutive escape characters (representing the escape character itself)
 |
 \\' #Eat the escape character followed by a quote (representing the escaped quote)
 |
 [^'\\]* #Other characters
 )*
 '
 )
 )*
 >
 .*? #Tag content
  #End tag
 ;x
 heredoc
 , $s, $matches, PREG_OFFSET_CAPTURE, $o);
 if ( $is_match ) {
 foreach ( $matches[0] as $match ) {
 $o0 = $match[1];
 #The label left boundary is intercepted when it crosses Right boundary of the target, exit
 if ( $o0 >= $o + $l ) break;
 $l0 = strlen($match[0]);
 #The right boundary of the label is within the right boundary of the intercepted target, continue
 if ( $o0 + $l0 < $o + $l ) continue;

 #The following is label cross-border processing
 $l = $o0 + $l0 - $o;
 break;
 }
 }
 return substr($s , $o, $l);
 }
 echo $str . chr(10);
 echo substr_remain_tag($str, 0, 20) . chr(10);