Practical Guide to PHP Regular Expressions: Efficient Data Collection Techniques Explained

1. Introduction to Regular Expressions

Regular expressions are a powerful tool for matching strings based on specific patterns. In PHP development, regex is widely used in scenarios such as data collection and format validation. Below are some commonly used regex examples:

1.1 Match Any Character

The dot . in regular expressions matches any single character except newline. For example, the expression '.' matches any single character:

<span class="fun">'.'</span>

1.2 Match Specific Character Sets

Square brackets [] match any one character inside them. For example:

<span class="fun">[abc]</span>

This matches the characters a, b, or c.

To match a range of characters, use a hyphen -, such as:

<span class="fun">[a-z]</span>

This matches all lowercase English letters.

1.3 Quantifiers

Quantifiers control how many times a character can appear. Common ones include:

• ? - matches the preceding character 0 or 1 time
• * - matches the preceding character 0 or more times
• + - matches the preceding character 1 or more times
• {n} - matches exactly n times
• {n,} - matches at least n times
• {n,m} - matches between n and m times

For example, to match 1 to 2 hexadecimal digits:

<span class="fun">[0-9a-fA-F]{1,2}</span>

This expression matches digits 0-9 and letters a-f (case insensitive), occurring once or twice.

2. Practical Use of Regular Expressions in PHP: Data Collection

2.1 Fetching Webpage Content with curl

PHP’s curl library allows convenient retrieval of web data. Here is an example requesting the homepage of Baidu:

$curl = curl_init('http://www.baidu.com');
curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
$html = curl_exec($curl);
curl_close($curl);
<p>echo $html;<br>

In this code, curl_init() initializes the curl session, curl_setopt() sets the option to return the content, curl_exec() executes the request, and finally the session is closed.

2.2 Extracting Links from the Webpage

After obtaining the HTML, you can use regex to extract specific content. For example, to extract all tags’ href attributes and link text:

preg_match_all('/<a href="(.*)" target="_blank">(.*)<\/a>/U', $html, $matches);
foreach ($matches[2] as $match) {
  echo $match . '\n';
}

This regex matches all qualifying tags and stores href attributes and link texts in the $matches array. Iterating over $matches[2] outputs all link texts.

preg_match_all('/<img src="(.*)" width=.* height=.*>/U', $html, $matches);
foreach ($matches[1] as $match) {
  echo $match . '\n';
}