noodlehaus/fmt-test.php

## fmt-test.php
<?php
include './fmt.php';

$text = '';

if (strtolower($_SERVER['REQUEST_METHOD']) === 'post')
  $text = $_POST['text'];
?>
<!DOCTYPE html>
<html>
<head>
<title>fmt test</title>
<style>
textarea { width: 500px; height: 200px; }
</style>
</head>
<body>
<pre>
<?= htmlentities(fmt($text)) ?>
</pre>
<div>
<?= fmt($text) ?>
</div>
<form method="post">
<div><textarea name="text"><?= htmlentities($text) ?></textarea></div>
<div><input type="submit"><a href="test.php">Reset</a></div>
</body>
</html>

## fmt.php
<?php
function fmt($text) {

  $text = htmlentities($text);

  // bold
  $text = preg_replace_callback('@\*(.+)\*@U', function ($m) {
    return '<strong>'.trim($m[1]).'</strong>';
  }, $text);

  // images
  $text = preg_replace_callback(
    '@(?:&lt;&lt;)(https?:\/\/[^\s]+)(?:&gt;&gt;)@',
    function ($m) {
      return '<img src="'.filter_var($m[1], FILTER_SANITIZE_URL).'">';
    },
    $text
  );

  // text links
  $text = preg_replace_callback(
    '@\[(.+)\](?:&lt;)(https?:\/\/[^\s]+)(?:&gt;)@',
    function ($m) {
      return (
        '<a href="'.
        filter_var($m[2], FILTER_SANITIZE_URL).
        '" rel="nofollow">'.
        $m[1].
        '</a>'
      );
    },
    $text
  );

  // urls
  $text = preg_replace_callback(
    '@(?:&lt;)(https?:\/\/[^\s]+)(?:&gt;)@',
    function ($m) {
      return (
        '<a href="'.
        filter_var($m[1], FILTER_SANITIZE_URL).
        '" rel="nofollow">'.
        $m[1].
        '</a>'
      );
    },
    $text
  );

  // blockquote
  $text = preg_replace(
    '@</blockquote>\s<blockquote>@m',
    ' ',
    preg_replace_callback(
      '@^&gt;&gt;(.*)@m',
      function ($m) {
        return '<blockquote>'.trim($m[1]).'</blockquote>';
      },
      $text
    )
  );

  // unordered list
  $text = preg_replace(
    '@</ul>\s<ul>@m',
    '',
    preg_replace_callback('@^-(.*)@m', function ($m) {
      return '<ul><li>'.trim($m[1]).'</li></ul>';
    }, $text)
  );

  // ordered list
  $text = preg_replace(
    '@</ol>\s<ol>@m',
    '',
    preg_replace_callback('@^#(.*)@m', function ($m) {
      return '<ol><li>'.trim($m[1]).'</li></ol>';
    }, $text)
  );

  // paragraph
  $text = preg_replace_callback('@^(.+)(\r?\n)?@m', function ($m) {
    $line = trim($m[1]);
    return (strlen($line) ? '<p>'.$line.'</p>' : '');
  }, $text);

  // remove incorrect nesting
  $text = preg_replace('@<p><(ol|ul|blockquote)>@', '<\\1>', $text);
  $text = preg_replace('@</(ol|ul|blockquote)></p>@', '</\\1>', $text);

  return $text;
}
?>
	<?php
	include './fmt.php';

	$text = '';

	if (strtolower($_SERVER['REQUEST_METHOD']) === 'post')
	$text = $_POST['text'];
	?>
	<!DOCTYPE html>
	<html>
	<head>
	<title>fmt test</title>
	<style>
	textarea { width: 500px; height: 200px; }
	</style>
	</head>
	<body>
	<pre>
	<?= htmlentities(fmt($text)) ?>
	</pre>
	<div>
	<?= fmt($text) ?>
	</div>
	<form method="post">
	<div><textarea name="text"><?= htmlentities($text) ?></textarea></div>
	<div><input type="submit"><a href="test.php">Reset</a></div>
	</body>
	</html>
	<?php
	function fmt($text) {

	$text = htmlentities($text);

	// bold
	$text = preg_replace_callback('@\(.+)\@U', function ($m) {
	return '<strong>'.trim($m[1]).'</strong>';
	}, $text);

	// images
	$text = preg_replace_callback(
	'@(?:<<)(https?:\/\/[^\s]+)(?:>>)@',
	function ($m) {
	return '<img src="'.filter_var($m[1], FILTER_SANITIZE_URL).'">';
	},
	$text
	);

	// text links
	$text = preg_replace_callback(
	'@\[(.+)\](?:<)(https?:\/\/[^\s]+)(?:>)@',
	function ($m) {
	return (
	'<a href="'.
	filter_var($m[2], FILTER_SANITIZE_URL).
	'" rel="nofollow">'.
	$m[1].
	'</a>'
	);
	},
	$text
	);

	// urls
	$text = preg_replace_callback(
	'@(?:<)(https?:\/\/[^\s]+)(?:>)@',
	function ($m) {
	return (
	'<a href="'.
	filter_var($m[1], FILTER_SANITIZE_URL).
	'" rel="nofollow">'.
	$m[1].
	'</a>'
	);
	},
	$text
	);

	// blockquote
	$text = preg_replace(
	'@</blockquote>\s<blockquote>@m',
	' ',
	preg_replace_callback(
	'@^>>(.*)@m',
	function ($m) {
	return '<blockquote>'.trim($m[1]).'</blockquote>';
	},
	$text
	)
	);

	// unordered list
	$text = preg_replace(
	'@</ul>\s<ul>@m',
	'',
	preg_replace_callback('@^-(.*)@m', function ($m) {
	return '<ul><li>'.trim($m[1]).'</li></ul>';
	}, $text)
	);

	// ordered list
	$text = preg_replace(
	'@</ol>\s<ol>@m',
	'',
	preg_replace_callback('@^#(.*)@m', function ($m) {
	return '<ol><li>'.trim($m[1]).'</li></ol>';
	}, $text)
	);

	// paragraph
	$text = preg_replace_callback('@^(.+)(\r?\n)?@m', function ($m) {
	$line = trim($m[1]);
	return (strlen($line) ? '<p>'.$line.'</p>' : '');
	}, $text);

	// remove incorrect nesting
	$text = preg_replace('@<p><(ol\|ul\|blockquote)>@', '<\\1>', $text);
	$text = preg_replace('@</(ol\|ul\|blockquote)></p>@', '</\\1>', $text);

	return $text;
	}
	?>