How can regular expressions be used in PHP to search for specific patterns in a text file?

Regular expressions can be used in PHP to search for specific patterns in a text file by using functions like preg_match() or preg_match_all(). These functions allow you to define a regular expression pattern that describes the text you want to search for, and then search for that pattern within the text file. By using regular expressions, you can search for complex patterns, such as email addresses or phone numbers, with ease.

// Open the text file
$file = fopen("example.txt", "r");

// Read the contents of the file
$text = fread($file, filesize("example.txt"));

// Define the regular expression pattern
$pattern = "/[0-9]{3}-[0-9]{3}-[0-9]{4}/";

// Search for the pattern in the text
if (preg_match_all($pattern, $text, $matches)) {
    echo "Phone numbers found: ";
    print_r($matches[0]);
} else {
    echo "No phone numbers found.";
}

// Close the file
fclose($file);