How to copy a piece of text into a textarea

Preg_match and preg_match_all use REGEX and return a boolean indicating whether the expression (default) was found. About Regular Expressions you can ask your questions here and test here / a>

As for the problem itself, you can do this:

<?php
// Pega o POST
$html = filter_input(INPUT_POST, 'htmlcode');

// Se encontrar algum <h1> no html...
if(preg_match('/<h1>(.*)<\/h1>/iU', $html, $match)){
    echo $match[1];
}

To get more than one occurrence, use preg_match_all , like this:

<?php
// Pega o POST
$html = filter_input(INPUT_POST, 'htmlcode');

// Se encontrar algum <h1> no html...
if(preg_match_all('/<h1>(.*)<\/h1>/iU', $html, $match)){
    // Pega as ocorrencias do conteúdo de <h1>
    $ocorrencias = $match[1];
    // Tem alguma ocorrência?
    if(!empty($ocorrencias)){
        // Lê uma a uma
        foreach($ocorrencias as $index => $ocorrencia){
            // Exibe cada uma das ocorrências do conteúdo de <h1>
            echo "$index - $ocorrencia <br>";
        }
    }
}

Explaining the modifiers

Note that in REGEX within preg_match() everything inside / is the default. what is outside are modifiers. The i f modifier indicates to ignore the case (lowercase) and the modifier U indicates that REGEX is not greedy, that is, in ". *" it will stop picking up when it finds the next pattern that is </h1> . Note tb that in <\/h1> has been escaped the bar for REGEX not to confuse with the closing bar.

You can see more about the modifiers here .

Do not parse HTML using preg_match .

Use the DOMDocument class

Example:

<?php 

$html= "<p>Olá!</p>
<h1>Titulo H1</h1>
<h2>Titulo H2</h2>
<h3>Titulo H3</h3>";
// Cria um novo objeto DOM
$dom = new domDocument('1.0', 'utf-8'); 
// Carrega o HTML no Objeto
$dom->loadHTML($html); 
//remove espaços em branco
$dom->preserveWhiteSpace = false; 
$hTwo= $dom->getElementsByTagName('h2'); // Defina aqui a sua tag desejada
echo $hTwo->item(0)->nodeValue; 
//Irá retornar "Titulo H2";
?>

If I realize I'm printing $hTwo->item(0)->nodeValue where item(0) has the index of the item I'll get the nodeValue .

If there is more than one <h2> item on the page $hTwo->length will return the total of items with the same TagName on the page, then you can make a loop to print all these elements

Example:

<?php 
$html = "<p>Olá!</p>
    <h1>Titulo H1</h1>
    <h2>Titulo H2</h2>
    <h2>Titulo H2 2</h2>";
// Cria um novo objeto DOM
$dom = new domDocument('1.0', 'utf-8');
// Carrega o HTML no Objeto
$dom->loadHTML($html);
//remove espaços em branco
$dom->preserveWhiteSpace = false;
$hTwo = $dom->getElementsByTagName('h2'); // Defina aqui a sua tag desejada
for ($i = 0; $i < $hTwo->length; $i++) {
    echo $hTwo->item($i)->nodeValue."<br>";
}
//Irá retornar "Titulo H2<br>Titulo H2 2;
?>