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

• Giel Berkers: Thanks for your answer! I hadn't thought about using preg_replace_callback() for this and passing $levels bij reference.