Pattern matching using PHP and Regular Expressions

In modern programming languages, pattern matching is a very common operation. It is used to find parts of a string that match certain rules. In PHP, regular expressions are very convenient for pattern matching. In this article, we'll cover pattern matching using PHP and regular expressions.

First, we need to understand what a regular expression is. Regular expressions are a language used for pattern matching. It can be used to find, replace, validate and extract specific parts of a string. Regular expressions consist of some characters and special characters. These characters are used to match strings.

In PHP, we can use the preg_match() function to perform regular expression matching. Here is an example:

$string = "This is a test string.";
$pattern = "/test/i";
preg_match($pattern, $string, $matches);
print_r($matches);

In the above example, $string is the string to search for. $pattern is a regular expression. In this example, we use "/test/i" as the regular expression. This means we are looking for "test" in the string, where "i" means case insensitive. Finally, $matches is an array containing all matching results. In this case, it should output:

Array
(
    [0] => test
)

If no matching string is found, the $matches array will be empty.

Now, let’s take a look at the most basic characters in regular expressions.

• .: Match any character
• ^: Match the beginning of the string
• $: Match the end of the string
• []: Match Any character in brackets
• [a-z]: Matches any character between a - z
• ¹: Matches any character except a - z Any character outside
• (): Match the expression inside the brackets

The following is an example to match a zip code (postal code):

$string = "12345-6789";
$pattern = "/^[0-9]{5}(-[0-9]{4})?$/";
preg_match($pattern, $string, $matches);
print_r($matches);

In the above example, we used some different characters:

• ^: represents the beginning of the string
• [0-9]{5}: matches 5 numbers
• (-[0-9]{4})?: Matches an optional hyphen and 4 digits
• $: Indicates the end of the string

This regular expression can match the zip code in the following format:

• 12345
• 12345-6789

Now, we have learned enough knowledge, you can start writing a more complex script. Here is an example to find all PHP files in a folder and list all the functions and methods in them:

$dir = "./";
$files = scandir($dir);

foreach ($files as $file) {
    if (preg_match("/.php$/", $file)) {
        $content = file_get_contents($file);
        preg_match_all("/(function|public function|private function|protected function|method|public method|private method|protected method) ([a-zA-Z0-9_]+)/", $content, $matches);
        echo "In $file:
";
        foreach ($matches[2] as $match) {
            echo "- $match()
";
        }
    }
}

In the above example, we have used the preg_match_all() function. This function is similar to the preg_match() function, except that it looks for all matches instead of just the first match. We also used a regular expression to match all functions and methods. In this example, we will only list public, private and protected functions and methods.

We have seen how to do pattern matching using PHP and regular expressions. This technique is very powerful and can be used for many different purposes, from simple string matching to complex data validation. If you haven't tried using regular expressions yet, we encourage you to take some time to learn them and start using them, as they can help you write more efficient and flexible code.

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1. a-z ↩

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