Haml.php

<?php
namespace Haml;

class Haml
{
    public static function getTree($data)
    {
        $root = $last_node = new Node('', -1);
        $re = '/^[\r\n]*([ \t]*)([^ \t]?(?:(?=.*?{[^}]*)\s*[^}]*}.*?|' .
              '(?:(?=.*?\s*\|\s*)(?:.*?\s*\|\s*)*|.*?)))(\s*?)[\r\n]*$/m';
        $indent_char = '';
        $not_indent  = '';
        $indent_amt  = 0;
        $line        = 0;
        while (!empty($data)) {
            $line++;
            if (!preg_match($re, $data, $matches)) {
                throw new Exception(sprintf(
                    'Parse error on line %d: ' .
                    'Unknown error',
                    $line
                ));
            }
            $data = \substr($data, \strlen($matches[0]) + \strlen($matches[4]));
            $matches[0] = ltrim($matches[2], "\r\n");
            if (empty($matches[0])) {
                continue;
            }
            $indent = $matches[1];
            if (count($root) == 0 && \strlen($indent) > 0) {
                throw new Exception(sprintf(
                    'Syntax error on line %d: ' .
                    'Indenting at the beginning of the document is illegal.',
                    $line
                ));
            }
            if (empty($indent_char)) {
                $indent_char = substr($indent, 0, 1);
                $indent_amt  = strlen($indent);
                $not_indent  = ($indent_char === ' ' ? "\t" : ' ');
            } else if (strpos($indent, $not_indent) !== false) {
                throw new Exception(sprintf(
                    'Syntax error on line %d: ' .
                    'Indentation can\'t use both tabs and spaces.',
                    $line
                ));
            } else if (!is_int(strlen($indent) / $indent_amt)) {
                throw new Exception(sprintf(
                    'Syntax error on line %d: ' .
                    'Inconsistent indentation: ' .
                    '%d %s%s %s used for indentation, but the rest ' .
                    'of the document was indented using %d %s%s',
                    $line,
                    strlen($indent),
                    substr($indent, 0, 1) === ' ' ? 'space' : 'tab',
                    strlen($indent) != 1 ? 's' : '',
                    strlen($indent) != 1 ? 'were' : 'was',
                    $indent_amt,
                    $indent_char === ' ' ? 'space' : 'tab',
                    $indent_amt != 1 ? 's' : ''
                ));
            } else {
                $level      = strlen($indent);
                $last_level = $last_node->getIndent() * $indent_amt;
                $expected   = $indent_amt + $last_level;
                if ($level > $expected) {
                    throw new Exception(sprintf(
                        'Syntax error on line %d: ' .
                        'The line was indented %d levels deeper ' .
                        'than the previous line.',
                        $line,
                        ($level - $last_level) / $indent_amt
                    ));
                }
            }

            $indent = \strlen($indent);
            if ($indent_amt > 0) {
                $indent = $indent / $indent_amt;
            }
            $node = new Node(trim($matches[2]), $indent);
            while (!$node->hasParent()) {
                if ($node->getIndent() > $last_node->getIndent() || !$last_node->hasParent()) {
                    if (substr($last_node->getContent(), 0, 3) === '!!!') {
                        throw new Exception(sprintf(
                            'Syntax error on line %d: ' .
                            'Illegal nesting: ' .
                            'nesting within a header command is illegal.',
                            $line
                        ));
                    }
                    $last_node->addChild($node);
                    $last_node = $node;
                } else {
                    $last_node = $last_node->getParent();
                }
            }
            $line += preg_match_all('/(\r\n?|\n)/', $matches[0]);
            $last_node = $node;
        }
        return $root;
    }

    public static function render($data)
    {
        $tree = static::getTree($data);
        $iterator = new RecursiveIteratorIterator($tree, RecursiveIteratorIterator::CHILD_FIRST);
        foreach ($iterator AS $node) {
            echo $node->getIndent() . " => " . (string)$node . PHP_EOL;
        }
    }
}

Node.php

<?php
namespace Haml;

class Node implements \RecursiveIterator, \Countable
{
    protected $parent   = null;
    protected $children = [];
    protected $ptr      = 0;
    protected $content  = '';
    protected $indent   = 0;

    public function __construct($content, $indent)
    {
        $this->setContent($content);
        $this->setIndent($indent);
    }
    public function getContent()
    {
        return $this->content;
    }
    public function setContent($content)
    {
        $this->content = strval($content);
    }
    public function getIndent()
    {
        return $this->indent;
    }
    public function setIndent($indent)
    {
        $this->indent = intval($indent);
    }
    public function getParent()
    {
        return $this->parent;
    }
    public function setParent(Node $node)
    {
        $this->parent = $node;
    }
    public function hasParent()
    {
        return $this->parent instanceof Node;
    }
    public function addChild(Node $node)
    {
        $node->setParent($this);
        return \array_push($this->children, $node);
    }
    public function getChildren()
    {
        return $this->current();
    }
    public function hasChildren()
    {
        return count($this->current()) > 0;
    }
    public function current()
    {
        return $this->children[$this->key()];
    }
    public function key()
    {
        $keys = \array_keys($this->children);
        return $keys[$this->ptr];
    }
    public function next()
    {
        $this->ptr++;
    }
    public function rewind()
    {
        $this->ptr = 0;
    }
    public function valid()
    {
        $keys = \array_keys($this->children);
        return isset($keys[$this->ptr]);
    }
    public function count()
    {
        return count($this->children);
    }
    public function __toString()
    {
        return $this->getContent();
    }
}

output.txt

0 => !!! 1.1
0 => %html
 1 => %head
  2 => %meta{ :http-equiv => 'Content-Type',
            :content => 'application/xhtml+xml;charset=utf-8' }
  2 => - if ($title)
   3 => %title= $title
  2 => - else
   3 => %title= $pagename
  2 => %p Some text            |
         And some more text |
         And yet more text  |
 1 => %body
  2 => #header
   3 => %h1 Example page
   3 => - if ($slogan)
    4 => %span= $slogan
  2 => %p Some text            |
         And some more text |
         And yet more text  |
  2 => #content
   3 => %table.config.list
    4 => %tr
     5 => %th ID
     5 => %th Name
     5 => %th Value
    4 => - foreach ($config as $c)
     5 => %tr[$c]
      6 => %td= $c->ID
      6 => %td= $c->name
      6 => %td= $c->value
  2 => #footer
   3 => %span.author Random Hacker

test.haml

!!! 1.1
%html
  %head
    %meta{ :http-equiv => 'Content-Type',
            :content => 'application/xhtml+xml;charset=utf-8' }
    - if ($title)
      %title= $title
    - else
      %title= $pagename
    %p Some text            |
         And some more text |
         And yet more text  |
  %body
    #header
      %h1 Example page
      - if ($slogan)
        %span= $slogan
    %p Some text            |
         And some more text |
         And yet more text  |
    #content
      %table.config.list
        %tr
          %th ID
          %th Name
          %th Value
        - foreach ($config as $c)
          %tr[$c]
            %td= $c->ID
            %td= $c->name
            %td= $c->value
    #footer
      %span.author Random Hacker