Basic crawler tutorial: parsing HTML using PHP and regular expressions

With the rapid development of the Internet, we increasingly need to extract large amounts of data from web pages as the basis for our daily life and work, which requires the use of crawler tools. This article will introduce how to use PHP and regular expressions to parse data based on HTML documents.

1. Overview of crawlers

Before understanding crawlers in depth, we need to know what a crawler is. The so-called crawler is a network data collection tool that can automatically collect information from the Internet and perform processing such as screening, integration, and analysis, and finally form a certain data set. Crawlers are mainly used in fields such as data mining, business competitive intelligence collection, and academic research.

2. Use PHP to parse HTML

Before we create a crawler, we need to understand how to parse data from HTML documents. As a server-side scripting language, PHP has a very convenient HTML parsing function. Commonly used HTML parsing libraries include simple_html_dom, phpQuery, etc. These libraries provide convenience for us to use CSS selectors and jQuery way syntax in PHP, and we can easily parse data from HTML files.

Before introducing how to use regular expressions to parse HTML, let's first take a look at how to use simple_html_dom for HTML parsing. This is a very convenient and easy-to-use HTML parser. You only need to use the following code :

require_once('simple_html_dom.php');
$html = file_get_html('http://example.com/');
echo $html->find('title',0)->plaintext;

The above code can obtain the content of the title tag in the specified URL (http://example.com/) and output it. $html is the HTML DOM object.

3. Use regular expressions to parse HTML

Regular expression is a method of describing text patterns (string patterns) and is a general pattern matching tool. Using regular expressions, we can easily perform various complex operations on text, including data search, replacement, separation, etc. When parsing HTML data, we often need to use regular expressions to match and extract specific tags, attributes, or content.

The following is a simple example for parsing the img tag in HTML code:

$match = preg_match_all('/<img.*?src=["|']?(.*?)["|']?s.*?>/i', $html, $out_img, PREG_SET_ORDER);
foreach ($out_img as $img_item) {
    echo $img_item[1];    
}

The above code uses the preg_match_all function to match the a1f02c36ba31691bcfe87b2722de723b tag in HTML through regular expressions, and The src attribute value is extracted and output to the screen.

4. Crawler implementation

Based on the above code example, we can slightly modify it and combine it with the curl library to implement a simple crawler. The following code can download the specified page and extract all link addresses in it:

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, 'https://www.example.com/');
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);//将页面转化成字符串，而不是直接输出
$html = curl_exec($ch);
curl_close($ch);

preg_match_all('/<a.*?href=["|']?(.*?)["|']?s.*?>/i', $html, $out_links, PREG_SET_ORDER);

foreach ($out_links as $link_item) {
    echo $link_item[1].PHP_EOL;//输出链接地址
}

In the above code, we use the curl library to obtain the web page source code. The PREG_SET_ORDER parameter indicates matching according to the output order of the regular expression. This crawler can complete simple link extraction functions. Of course, we can extend it by combining other regular expression patterns to meet more needs.

5. Summary

The above is the basic knowledge of how to use PHP and regular expressions to parse HTML documents. In actual work, we need to choose different parsing methods based on actual needs and web page structures, and appropriately combine other tools and libraries to complete complex data parsing tasks.

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