Advanced regex features for Markdown links, example

regex-markdown-example.php

<?php

$text = 'Text with two links [inline1](https://www.nrk.no) and [inline2](https://www.yr.no), may occur.';

$regex = <<<'REGEX'
(?<text_group>                  # Text group, including square brackets
  \[
    (?>                         # (?> defines an atomic group, this is a performance improvement when using recursion
      [^\[\]]+                  # Look for any char except closing square bracket
      |(?&text_group)           # OR: find recursively an other pattern with opening and closing square brackets
    )*
  \]
)
(?:
  \(
    (?<url>\S*?)                # URL: non-greedy non-whitespace characters
    (?:
      [ ]
      "
        (?<title>
          (?:[^"]|(?<=\\)")*?   # Title without double quotes around
        )
      "
    )?
  \)
)
REGEX;

preg_match_all("`$regex`x", $text, $matches, PREG_SET_ORDER);

var_dump(json_encode($matches, JSON_PRETTY_PRINT));