PHP 정규식의 사용법은 무엇입니까?

<?php
$array=array(1,2,3.4,53,7.9,.4);
$fl_array=preg_grep("/^(\d+)?\.\d+$/",$array);
print_r($fl_array);

<?php
$userinfo = "Name: <b>PHP</b> <br> Title: <b>Programming Language</b>";
preg_match_all ("/<b>(.*)<\/b>/U", $userinfo, $pat_array);
print_r($pat_array[0]);
?>

<?php
//\\2是一个后向引用的示例. 这会告诉pcre它必须匹配正则表达式中第二个圆括号(这里是([\w]+))
//匹配到的结果. 这里使用两个反斜线是因为这里使用了双引号.
$html = "<b>bold text</b><a href=howdy.html>click me</a>";
 
preg_match_all("/(<([\w]+)[^>]*>)(.*?)(<\/\\2>)/", $html, $matches, PREG_SET_ORDER);
 
foreach ($matches as $val) {
    echo "matched: " . $val[0] . "\n";
    echo "part 1: " . $val[1] . "\n";
    echo "part 2: " . $val[2] . "\n";
    echo "part 3: " . $val[3] . "\n";
    echo "part 4: " . $val[4] . "\n\n";
}
?>