wpautop adds a closing p-tag without an opening p-tag

[TBarregren]

Following code result in ill-formed HTML.

<?php
$pee = <<<EOT
<div>
This is a paragraph.

This is another paragraph.
</div>
EOT;

echo wpautop($pee);

This is the output:

<div>
This is a paragraph.</p>
<p>This is another paragraph.
</p></div>

As you can see, the first paragraph lacks an opening <p>.

[TBarregren]

If it is to any help, this is my function.php code to solve this (and related) problem(s).

<?php
add_filter('the_content', function ($html) {
  
  $html_pos = 0;
  $last_html_pos = strlen($html) - 1;

  $opening_pos = null;
  $text_pos = null;
  
  // See http://w3c.github.io/html/single-page.html#elementdef-p
  // and http://w3c.github.io/html/single-page.html#kinds-of-content-phrasing-content
  $self_closing_tags_allowed_in_p = explode(' ', 'area br embed img input link wbr');
  $not_self_closing_tags_allowed_in_p = explode(' ', 'a abbr audio b bdi bdo button canvas cite code data datalist del dfn em i iframe ins kbd label map mark math meter noscript object output picture progress q ruby s samp script select small span strong sub sup svg template textarea time u var video text');


  while (($html_pos = strpos($html, '<', $html_pos)) !== false) {
    if (substr($html, $html_pos, 4) === '<!--') {
      $html_pos = strpos($html, '-->', $html_pos + 4);
      if ($html_pos === false) return $html; // Too messy, don't do anything further.
      $html_pos += 3;
    } 
    elseif ($opening_pos === null) {
      if (substr($html, $html_pos, 4) === '<pre') {
        $html_pos = strpos($html, '</pre>', $html_pos + 6);
        if ($html_pos === false) return $html; // Too messy, don't do anything further.
        $html_pos += 6;
      }
      elseif (strtolower(substr($html, $html_pos, 2)) === '<p') {
        $opening_pos = $html_pos;
        $html_pos = strpos($html, '>', $html_pos + 2);
        if ($html_pos === false) return $html; // Too messy, don't do anything further.
        $text_pos = ++$html_pos;
      }
      elseif (strtolower(substr($html, $html_pos, 4)) === '</p>') {
        // See https://core.trac.wordpress.org/ticket/37672
        $html = substr($html, 0, $html_pos) . substr($html, $html_pos + 4);
        $last_html_pos -= 4;
      }
      else {
        $html_pos += 1;
      }
    }
    else {
      if (strtolower(substr($html, $html_pos, 4)) === '</p>') {
        if (trim(substr($html, $text_pos, $html_pos - $text_pos)) === '') {
          $html = substr($html, 0, $opening_pos) . substr($html, $html_pos + 4);
          $html_pos = $opening_pos;
          $last_html_pos = strlen($html) - 1;                
        }
        else {
          $html_pos += 4;
        }
        $opening_pos = null;
      }
      else {
        $tag_end_pos = $html_pos + 1;
        while ($html{$tag_end_pos} !== '>' && !ctype_space($html{$tag_end_pos})) {
          ++$tag_end_pos;
        }
        $tag = strtolower(substr($html, $html_pos + 1, $tag_end_pos - $html_pos - 1));
        if (in_array($tag, $self_closing_tags_allowed_in_p)) {
          $html_pos = strpos($html, '>', $tag_end_pos);
          if ($html_pos === false) return $html; // Too messy, don't do anything further.
          $html_pos += 1;
        }
        elseif (in_array($tag, $not_self_closing_tags_allowed_in_p)) {
          $html_pos = strpos($html, "</$tag", $tag_end_pos);
          if ($html_pos === false) return $html; // Too messy, don't do anything further.
          $html_pos += 1;
        }
        else {
          $tag_end_pos = strpos($html, "</$tag>", $tag_end_pos);
          if ($tag_end_pos === false) return $html; // Too messy, don't do anything further.
          $tag_end_pos += strlen("</$tag>");
          $html = substr($html, 0, $html_pos) . '</p>' . substr($html, $html_pos, $tag_end_pos - $html_pos) . '<p>' . substr($html, $tag_end_pos);
        }
      }
    }
  }
  if ($opening_pos !== null) {
    $html .= '</p>';
  }

  return $html;

});

[MattyRob]

@TBarregren

Thank for your report - I can reproduce this with a unit test and I think I've also managed to create a fix in the 'wpautop()' function. Patch and Unit test attached above.

[desrosj]

@MattyRob are you able to refresh 37672.diff​ to apply cleanly to trunk? Having a unit test demonstrating the issue may help others dive in.

Trac ticket: https://core.trac.wordpress.org/ticket/37672

Code used in testing:

function wpautop_test_shortcode() {
    $text = <<<EOT
<div>
This is a paragraph.

This is another paragraph.
</div>
EOT;

    return wpautop($text);
}
add_shortcode('wpautop_test', 'wpautop_test_shortcode');

### Before:

### After:

[ozgursar]

Patch Testing Report

Patch Tested: ​https://github.com/WordPress/wordpress-develop/pull/9262

Environment

• WordPress: 7.0-alpha-61215-src
• PHP: 8.2.29
• Server: nginx/1.29.4
• Database: mysqli (Server: 8.4.7 / Client: mysqlnd 8.2.29)
• Browser: Chrome 144.0.0.0
• OS: macOS
• Theme: Twenty Twenty-One 2.7
• MU Plugins: None activated
• Plugins:
  • Classic Editor 1.6.7
  • Code Snippets 3.9.4
  • Test Reports 1.2.1

Steps taken

1. Create a sample shortcode with the following code using Code Snippets plugin or functions.php

function wpautop_test_shortcode() {
    $text = <<<EOT
<div>
This is a paragraph.

This is another paragraph.
</div>
EOT;

    return wpautop($text);
}
add_shortcode('wpautop_test', 'wpautop_test_shortcode');

2. Create a new page / post
3. Add the shortcode [wpautop_test] to the page content and publish
4. View the post in frontend and check the source code
5. Apply the patch
6. View the post in frontend and check the source code again
7. ✅ Patch is solving the problem

Expected result

• After applying the patch p tags are correctly added to the shortcode content.

Screenshots/Screencast with results

Before:

After: