PHP 函数如何实现正则表达式匹配？

preg_match($regex, $string, $matches);
preg_match_all($regex, $string, $matches);

$string = 'The quick brown fox jumps over the lazy dog';
$regex = '/fox/';
preg_match($regex, $string, $matches);
var_dump($matches);

array(1) {
  [0]=>
  string(3) "fox"
}

preg_replace($regex, $replacement, $string);

$string = 'The quick brown fox jumps over the lazy dog';
$regex = '/fox/';
$replacement = 'cat';
$result = preg_replace($regex, $replacement, $string);
var_dump($result);

string(34) "The quick brown cat jumps over the lazy dog"

preg_split($regex, $string, $limit);

$string = 'The quick brown fox jumps over the lazy dog';
$regex = '/ /';
$limit = 3;
$array = preg_split($regex, $string, $limit);
var_dump($array);

array(3) {
  [0]=>
  string(4) "The"
  [1]=>
  string(6) "quick"
  [2]=>
  string(6) "brown"
}

function validateEmail($email) {
  $regex = '/^[a-z0-9._%+-]+@(?:[a-z0-9-]+\.)+[a-z]{2,6}$/';
  return preg_match($regex, $email);
}

function searchText($text, $keyword) {
  $regex = '/' . preg_quote($keyword, '/') . '/';
  return preg_match($regex, $text);
}