markdown-like, but not. function for formatting text.

nomad.php

<?php
function nomad($text) {

  $text = htmlentities($text);

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

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

  // ordered list
  $text = preg_replace(
    '@</ol>\s<ol>@m',
    '',
    preg_replace_callback('@^\s*#(.*)@m', function ($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 trim($text);
}
?>