PHP Regular Expressions

<?php
$str = "The price is $42.50 and $18.99";

// preg_match — find first match
if (preg_match('/\$(\d+\.\d{2})/', $str, $matches)) {
    echo "First price: " . $matches[0]; // $42.50
    echo "Amount: "      . $matches[1]; // 42.50
}

// preg_match_all — find all matches
$count = preg_match_all('/\$(\d+\.\d{2})/', $str, $all);
echo "Found $count prices\n";
print_r($all[1]); // [42.50, 18.99]

// Validate email
$email = "user@example.com";
if (preg_match('/^[a-zA-Z0-9._%+\-]+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,}$/', $email)) {
    echo "Valid email";
}

// Case-insensitive match (i modifier)
if (preg_match('/hello/i', "Hello World")) {
    echo "Found (case-insensitive)";
}
?>

<?php
// preg_replace — replace pattern with string
$text = "Hello   World   PHP";
$clean = preg_replace('/\s+/', ' ', $text); // collapse whitespace
echo $clean; // Hello World PHP

// Replace with backreference
$date = "2024-01-15";
$formatted = preg_replace('/(\d{4})-(\d{2})-(\d{2})/', '$3/$2/$1', $date);
echo $formatted; // 15/01/2024

// Remove HTML tags
$html = "<p>Hello <b>World</b></p>";
echo preg_replace('/<[^>]+>/', '', $html); // Hello World

// preg_split — split by pattern
$csv = "one, two,  three ,four";
$parts = preg_split('/\s*,\s*/', trim($csv));
print_r($parts); // [one, two, three, four]

// Split on multiple delimiters
$str = "apple;banana|cherry,date";
$fruits = preg_split('/[;|,]/', $str);
print_r($fruits); // [apple, banana, cherry, date]
?>

<?php
// Common patterns
$patterns = [
    'digits only'     => '/^\d+$/',
    'alphanumeric'    => '/^[a-zA-Z0-9]+$/',
    'phone (US)'      => '/^\+?1?\s?\(?\d{3}\)?[\s.\-]?\d{3}[\s.\-]?\d{4}$/',
    'URL'             => '/^https?:\/\/[\w\-]+(\.[\w\-]+)+([\w\-\._~:\/\?#\[\]@!\$&\'\(\)\*\+,;=.]+)?$/',
    'IPv4'            => '/^(\d{1,3}\.){3}\d{1,3}$/',
    'hex color'       => '/^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/',
    'strong password' => '/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[\W_]).{8,}$/',
];

$tests = [
    'digits only'  => '12345',
    'alphanumeric' => 'Hello123',
    'hex color'    => '#FF5733',
];

foreach ($tests as $name => $value) {
    $result = preg_match($patterns[$name], $value) ? "PASS" : "FAIL";
    echo "$name ('$value'): $result\n";
}

// Modifiers:
// i — case insensitive
// m — multiline (^ and $ match line start/end)
// s — dotall (. matches newline too)
// x — extended (allow whitespace and comments)
// u — unicode

// Multiline example
$text = "Line 1\nLine 2\nLine 3";
preg_match_all('/^Line \d+$/m', $text, $matches);
print_r($matches[0]); // [Line 1, Line 2, Line 3]
?>