Methods and techniques for matching nested HTML tags with PHP regular expressions

Reprint please indicate the source: http://blog.csdn.net/donglynn/article/details/35788879

Regular expressions are a very useful programming skill. Generally, it is easy to grab a piece of information from an HTML page, such as a <title> title </title>. However, we tend to crawl the specific contents of multiple <div></div> blocks in a list page, and the <div></div> blocks are nested, and we crawl with each repeating <div Multiple information in a ></div> block. At the same time, the Web page source file differs from the generic string, and there is a large number of carriage returns, line feeds, and tabs, all of which result in a matching failure. Beginners are often unable to determine which link is the problem, and see highly skilled regular expressions will feel very frustrated, leading to the solution of the abandonment problem.

After the author of several days of research, and finally explore the following methods and skills, welcome to exchange corrections.

Look at the following note points and steps:

1. Note/must be escaped to \/, otherwise it will be an error

Preg_match_all () [Function.preg-match-all]: Unknown modifier

2. Regular expressions use single quotes ' and/As the bounds of start and end, such as '/reg partten/', in which the double quotes in the regular expression "do not have to escape

Like what

$partten = '/<div class= ' Goods_item "><a href=" ([^<>]+) "target=" _blank ">]+) "alt=" ([^<>]+) "Width=" "height=" "\/>/";

3. You need to remove all line breaks, tabs, carriage returns, and so on, for easy to read HTML source files due to the existence of these symbols can not match.

$str =preg_replace ("/[\t\n\r]+/", "", $str);

4. The matching information we are interested in is usually the value of the attribute in the HTML element, so you want to remove the <>, otherwise you will only match all the information before the last one.

For example, for $string= "<div><a href=" 1.jpg "></a></div><div><a href=" 2.jpg "></a> </div><div><a href= "3.jpg" ></a></div> ",

$partten = '/<div><a href= ' (. +) "/"; The match result is: 1.jpg "></a></div><div><a href=" 2.jpg " ></a></div><div><a href= "3.jpg" ></a></div>

This is because the given regular expression does not qualify the matching range just the first hyperlink <a href= "1.jpg" ></a>.

Therefore, to match the href attribute of the above three hyperlinks, you need to qualify the above matching in the <a href= "1.jpg" >, the method is very simple, replace (. +) with ([^<>]+), you can. That is, the match does not contain the next occurrence of the <>, thus qualifying the match within the same HTML tag

Do the above, you can completely ignore the HTML tags nested nesting problem, so crawled to a page of all the DIV repeat block we are interested in the content, attached to an example.

<?//Matched HTML code $html = ' <div class= ' goods ' > <a href= ' http://url1111 ' target= ' _blank ' >  </a> </div> <div class = "goods" > <a href= "http://url2222" target= "_blank" >  </a> </div> <div class=" goods "> <a href=" http://url3333 "target=" _blank "

>  </a> </div>";

Remove lines, tables and other special characters, you can echo to see the effect $html =preg_replace ("/[\t\n\r]+/", "", $html); Match expression, note two points, one is included in '//' inside, then/to do escape processing into \/$partern = '/<div class= ' goods "><a href=" ([^<>]+) "target=" _ Blank ">]+) "alt=" ([^<>]+) "Width=" "height=" "\/><\/a><\/"

div>/'; 

Matching results preg_match_all ($partern, $html, $result); 
Print results var_dump ($result); ? 

Output, a total of 4 sub arrays, the first of which are matched to all of the items, and the following three sub arrays are the three matches in our matching expression:

Array (4) {[0]=> Array (3) {[0]=> string (144) "<div class=" goods "><a href=" http://url1111 "Targ et= "_blank" ></a></div" > "[1]=> string (144)" <div class= "goods" ><a href= "http://url2222" target= "_blank" ></a></div>" [2]=> string (144) "&L" T;div class= "goods" ><a href= "http://url3333" target= "_blank" ></a></div>"} [1]=> Array (3) {[0]=> string () http ://url1111 "[1]=> string" http://url2222 "[2]=> string" http://url3333 "} [2]=> arr Ay (3) {[0]=> string "http://1111.jpg" [1]=> string () "Http://2222.jpg" [2]=> Strin G () "Http://3333.jpg"} [3]=> ARray (3) {[0]=> string (7) "alt1111" [1]=> string (7) "alt2222" [2]=> string (7) "alt3333" }
}