Adding prefixes to headers in markdown

Question

I'm adding prefixes to headers in markdown using a script. For example, if I have:

# Hello

## World

### Let's add

## Some headers

### Yay!

# Foo

## Bar

I transform it to:

# 1 Hello

## 1.1 World

### 1.1.1 Let's add

## 1.2 Some headers

### 1.2.1 Yay!

# 2 Foo

## 2.1 Bar

I currently have the following working code, but it feels a bit hacky and prone for error. :

preg_match_all('/^(#+)\s(.*)$/m', $markdown, $headers);
$levelCount = [];
$currentLevel = 9999;
foreach ($headers[2] as $idx => $header) {
    $level = strlen($headers[1][$idx]);
    if ($level < $currentLevel) {
        // reset:
        for ($i = $level; $i != $currentLevel; $i += 1) {
            array_pop($levelCount);
        }
    }
    if (!isset($levelCount[$level])) {
        $levelCount[$level] = 1;
    } else {
        $levelCount[$level] += 1;
    }
    $prefix = implode('.', array_values($levelCount));
    $currentLevel = $level;
    $markdown = preg_replace(
        '/' . preg_quote($headers[0][$idx]) . '/',
        $headers[1][$idx] . ' ' . $prefix . ' ' . $header,
        $markdown,
        1
    );
}

What are your thoughts on this?

Answer 1

I may not be considering all fringe cases (please enlighten me if this breaks with any realistic input), but all of the logic can be packed into a single preg_replace_callback() call.

• I start by declaring a 1-dimensional array which will contain all numeric counters.

• My pattern ~^(#+)\K~m says:
  From the start of each line, capture one or more hash symbols as capture group #1, then restart the fullstring match . In doing so, my replacement string to be determined inside the custom function will not actually be replacing any characters, but adding new characters at the zero-length position marked by \K. This spares you needing to match the rest of the line and adding that to the replacement.

• Making $levels modifiable by reference with & means that $level will be updated with each iteration of the regex matches.

• After counting the elements in $levels and counting the length of the matched string, precise modifications can be done to $levels to provide the desired set of numbers.

• The return value is a space followed by the dot-imploded $levels array.

• array_slice() avoids poping in a loop.

• I don't think your array_values() call is necessary.

Code: (Demo)

$headers = <<<HEADERS
# Hello

## World

### Let's add

## Some headers

### Yay!

# Foo

## Bar

## Bar Again
HEADERS;

$levels = [];
echo preg_replace_callback(
        '~^(#+)\K~m',
        function($m) use (&$levels) {
            $hashes = strlen($m[1]);
            $index = $hashes - 1;
            $reduction = sizeof($levels) - $hashes;
            if (!isset($levels[$index])) {
                $levels[$index] = 1;
            } else {
                ++$levels[$index];
            }
            if ($reduction > 0) {
                $levels = array_slice($levels, 0, -$reduction);
            }
            return " " . implode('.', $levels);
        },
        $headers
    );

Output:

# 1 Hello

## 1.1 World

### 1.1.1 Let's add

## 1.2 Some headers

### 1.2.1 Yay!

# 2 Foo

## 2.1 Bar

## 2.2 Bar Again