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Nächste Überarbeitung		Vorhergehende Überarbeitung
ebnf:index [2009/01/20 00:55] – Vincent Tscherterebnf:index [2023/09/28 07:51] (aktuell) – angelegt Vincent Tscherter
Zeile 1: Zeile 1:
 +====== EBNF Parser & Syntax Diagram Renderer ======
 +The //EBNF Parser & Syntax Diagram Renderer// can be used stand-alone or as Dokuwiki-Plugin.
 +
 +===== Download and Install=====
 +  * https://www.dokuwiki.org/plugin:ebnf
 +  * Requirements: Make sure to have the [[http://www.php.net/manual/en/image.requirements.php|GD library]] ready :!:
 +===== How to use =====
 +
 +  <ebnf> "EBNF defined in itself" {
 +  syntax     = [ title ] "{" { production } "}" [ comment ].
 +  production = identifier "=" expression ( "." | ";" ) .
 +  expression = term { "|" term } .
 +  term       = factor { factor } .
 +  factor     = identifier
 +             | literal
 +             | "[" expression "]"
 +             | "(" expression ")"
 +             | "{" expression "}" .
 +  identifier = character { character } .
 +  title      = literal .
 +  comment    = literal .
 +  literal    = "'" character { character } "'"
 +             | '"' character { character } '"' .
 +  }</ebnf>
 +
 +
 +<ebnf> "EBNF defined in itself" {
 +  syntax     = [ title ] "{" { production } "}" [ comment ].
 +  production = identifier "=" expression ( "." | ";" ) .
 +  expression = term { "|" term } .
 +  term       = factor { factor } .
 +  factor     = identifier
 +             | literal
 +             | "[" expression "]"
 +             | "(" expression ")"
 +             | "{" expression "}" .
 +  identifier = character { character } .
 +  title      = literal .
 +  comment    = literal .
 +  literal    = "'" character { character } "'"
 +             | '"' character { character } '"' .
 +} </ebnf>
 +
 +The script is also available as stand-alone version under: http://karmin.ch/ebnf.php
 +  http://karmin.ch/ebnf.php?syntax="EBNF"{ hello = "Hello World". }"Some additional Comments"
 +Note: The stand-alone script supports also XML output
 +  http://karmin.ch/ebnf.php?syntax={ hello = "EBNF Hello World in XML". }&format=xml
 +
 +===== Syntax =====
 +  * See examples: [[examples]]
 +  * [[wp>EBNF]]
 +  * [[wp>Wirth_syntax_notation]]
 +
 +===== Todo & Ideas =====
 +  * Ellipse ... for enumerations. E.g. ''digits'': 
 +    * temp hack in v0.2: ''function render_node($node, $lefttoright)''<code php> ...
 +    else {
 +      if ($text!="...")
 +       rr($im, UNIT, 0, $w-UNIT-1, $h-1, UNIT/2, $black);
 +      imagestring($im, FONT, 2*UNIT, ($h-imagefontheight(FONT))/2, 
 +        $text, $text!="..."?$blue:$black);
 +    } ...</code>
 +  * Other fonts
 +  * Configurable dimensions
 +  * escape-chars and or unicode in terminals
 +  * Adding //Regexp// beside terminal and identifiers
 +  * using a lightweight syntaxtree instead DOM
 +  * using some standard parsing mechanism
 +  * support for comments & special-sequences ''? ... ?''
 +  * http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html
 +  * replace antialias function
 +
 +===== Code =====
 +<code php syntax.php>
 +<?php
 +/**
 + * Dokuwiki Plugin EBNF: Displays Syntax Diagrams
 + *
 + * Syntax: <ebnf> ebnf syntax </ebnf>
 + *
 + * @license    GPL3
 + * @author     Vincent Tscherter <vinent.tscherter@karmin.ch>
 + * @version    0.1
 + */
 +
 +if(!defined('DOKU_INC')) define('DOKU_INC',realpath(dirname(__FILE__).'/../../').'/');
 +if(!defined('DOKU_PLUGIN')) define('DOKU_PLUGIN',DOKU_INC.'lib/plugins/');
 +require_once(DOKU_PLUGIN.'syntax.php');
 +
 +class syntax_plugin_ebnf extends DokuWiki_Syntax_Plugin {
 +
 +    function getType(){
 +        return 'substition';
 +    }
 +
 +    function getSort(){
 +        return 999;
 +    }
 +
 +    function connectTo($mode) {
 +      $this->Lexer->addSpecialPattern('<ebnf>.*?</ebnf>',$mode,'plugin_ebnf');
 +    }
 +
 +    function handle($match, $state, $pos, &$handler){
 +       switch ($state) {
 +          case DOKU_LEXER_ENTER :
 +            break;
 +          case DOKU_LEXER_MATCHED :
 +            break;
 +          case DOKU_LEXER_UNMATCHED :
 +            break;
 +          case DOKU_LEXER_EXIT :
 +            break;
 +          case DOKU_LEXER_SPECIAL :
 +            break;
 +        }
 +        return array($match);
 +    }
 +
 +    function render($mode, &$renderer, $data) {
 +        if($mode == 'xhtml'){
 +            try {
 +             $text = substr($data[0], 6, strlen($data[0])-13);
 +             $text = preg_replace( "/[<>]+/", "", $text);
 +             $text = preg_replace( "/[\\n\\r\\t ]+/", " ", $text);
 +             $text = urlencode($text);
 +             $renderer->doc .= "<img src='".DOKU_URL."lib/plugins/ebnf/ebnf.php?syntax=$text' alt='$text'/>";            // ptype = 'normal'
 +            } catch (Exception $e) {
 +              $renderer->doc .= "<pre>".htmlentities($text)."\n".$e."</pre>";
 +            }
 +            return true;
 +        }
 +        return false;
 +    }
 +}
 +?>
 +</code>
 +<code php ebnf.php>
 +<?php
 +/*
 +    This program is free software: you can redistribute it and/or modify
 +    it under the terms of the GNU General Public License as published by
 +    the Free Software Foundation, either version 3 of the License, or
 +    (at your option) any later version.
 +
 +    This program is distributed in the hope that it will be useful,
 +    but WITHOUT ANY WARRANTY; without even the implied warranty of
 +    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 +    GNU General Public License for more details.
 +
 +    You should have received a copy of the GNU General Public License
 +    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 +
 +    Vincent Tscherter, tscherter@karmin.ch, Solothurn, 2009-01-18
 +
 +    2009-01-18 version 0.1 first release
 +    2009-01-02 version 0.2
 +      - title und comment literal added
 +      - ";" als terminator-symbol added
 +*/
 +error_reporting(E_ALL|E_STRICT);
 +
 +//
 +define('META', 'xis/ebnf v0.2 https://www.dokuwiki.org/plugin:ebnf gpl3');
 +
 +// parser
 +define('EBNF_OPERATOR_TOKEN', 1);
 +define('EBNF_LITERAL_TOKEN', 2);
 +define('EBNF_WHITESPACE_TOKEN', 3);
 +define('EBNF_IDENTIFIER_TOKEN', 4);
 +
 +// rendering
 +define('FONT', 3);
 +define('UNIT', 10);
 +define('AW', 3);
 +
 +// lexemes
 +$ebnf_lexemes[] = array( 'type' => EBNF_OPERATOR_TOKEN, 'expr' => '[={}()|.;[\]]' );
 +$ebnf_lexemes[] = array( 'type' => EBNF_LITERAL_TOKEN,  'expr' => "\"[^\"]*\"" );
 +$ebnf_lexemes[] = array( 'type' => EBNF_LITERAL_TOKEN,  'expr' => "'[^']*'" );
 +$ebnf_lexemes[] = array( 'type' => EBNF_IDENTIFIER_TOKEN,  'expr' => "[a-zA-Z0-9_-]+" );
 +$ebnf_lexemes[] = array( 'type' => EBNF_WHITESPACE_TOKEN,  'expr' => "\\s+" );
 +
 +// input example
 +$input = <<<EOD
 +"EBNF defined in itself" {
 +  syntax     = [ title ] "{" { rule } "}" [ comment ].
 +  rule       = identifier "=" expression ( "." | ";" ) .
 +  expression = term { "|" term } .
 +  term       = factor { factor } .
 +  factor     = identifier
 +             | literal
 +             | "[" expression "]"
 +             | "(" expression ")"
 +             | "{" expression "}" .
 +  identifier = character { character } .
 +  title      = literal .
 +  comment    = literal .
 +  literal    = "'" character { character } "'"
 +             | '"' character { character } '"' .
 +}
 +EOD;
 +
 +if (isset($_GET['syntax'])) {
 +  $input = $_GET['syntax'];
 +  $input = stripslashes($input);
 +}
 +
 +$format = "png";
 +if (isset($_GET['format'])) $format = $_GET['format'];
 +
 +try {
 +  $tokens = ebnf_scan($input, true);
 +  $dom = ebnf_parse_syntax($tokens);
 +  if ($format == 'xml') {
 +    header('Content-Type: application/xml');
 +    echo $dom->saveXML();
 +  } else {
 +    render_node($dom->firstChild, true);
 +  }
 +} catch (Exception $e) {
 +    header('Content-Type: text/plain');
 +    $dom = new DOMDocument();
 +    $syntax = $dom->createElement("syntax");
 +    $syntax->setAttribute('title', 'EBNF - Syntax Error');
 +    $syntax->setAttribute('meta', $e->getMessage());
 +    $dom->appendChild($syntax);
 +    render_node($dom->firstChild, true);
 +}
 +
 +function rr($im, $x1, $y1, $x2, $y2, $r, $black){
 +  imageline($im, $x1+$r, $y1, $x2-$r, $y1, $black);
 +  imageline($im, $x1+$r, $y2, $x2-$r, $y2, $black);
 +  imageline($im, $x1, $y1+$r, $x1, $y2-$r, $black);
 +  imageline($im, $x2, $y1+$r, $x2, $y2-$r, $black);
 +  imagearc($im, $x1+$r, $y1+$r, 2*$r, 2*$r, 180, 270, $black);
 +  imagearc($im, $x2-$r, $y1+$r, 2*$r, 2*$r, 270, 360, $black);
 +  imagearc($im, $x1+$r, $y2-$r, 2*$r, 2*$r, 90, 180, $black);
 +  imagearc($im, $x2-$r, $y2-$r, 2*$r, 2*$r, 0, 90, $black);
 +}
 +
 +function create_image($w, $h) {
 +  global $white, $black, $blue, $red, $green, $silver;
 +  $im = imagecreatetruecolor($w, $h) or die("no img");
 +  imageantialias($im, true);
 +  $white = imagecolorallocate ($im, 255, 255, 255);
 +  $black = imagecolorallocate ($im, 0, 0, 0);
 +  $blue = imagecolorallocate ($im, 0, 0, 255);
 +  $red = imagecolorallocate ($im, 255, 0, 0);
 +  $green = imagecolorallocate ($im, 0, 200, 0);
 +  $silver = imagecolorallocate ($im, 127, 127, 127);
 +  imagefilledrectangle($im, 0,0,$w,$h,$white);
 +  return $im;
 +}
 +
 +function arrow($image, $x, $y, $lefttoright) {
 +  global $white, $black;
 +  if (!$lefttoright)
 +    imagefilledpolygon($image,
 +      array($x, $y-UNIT/3, $x-UNIT, $y, $x, $y+UNIT/3), 3, $black);
 +  else
 +    imagefilledpolygon($image,
 +      array($x-UNIT, $y-UNIT/3, $x, $y, $x-UNIT, $y+UNIT/3), 3, $black);
 +}
 +
 +
 +function render_node($node, $lefttoright) {
 +  global $white, $black, $blue, $red, $green, $silver;
 +  if ($node->nodeName=='identifier' || $node->nodeName=='terminal') {
 +    $text = $node->getAttribute('value');
 +    $w = imagefontwidth(FONT)*(strlen($text)) + 4*UNIT;
 +    $h = 2*UNIT;
 +    $im = create_image($w, $h);
 +
 +    if ($node->nodeName!='terminal') {
 +        imagerectangle($im, UNIT, 0, $w-UNIT-1, $h-1, $black);
 +      imagestring($im, FONT, 2*UNIT, ($h-imagefontheight(FONT))/2,   $text, $red);
 +    } else {
 +      if ($text!="...")
 +       rr($im, UNIT, 0, $w-UNIT-1, $h-1, UNIT/2, $black);
 +      imagestring($im, FONT, 2*UNIT, ($h-imagefontheight(FONT))/2,
 +        $text, $text!="..."?$blue:$black);
 +    }
 +    imageline($im,0,UNIT, UNIT, UNIT, $black);
 +    imageline($im,$w-UNIT,UNIT, $w+1, UNIT, $black);
 +    return $im;
 +  } else if ($node->nodeName=='option' || $node->nodeName=='loop') {
 +    if ($node->nodeName=='loop')
 +      $lefttoright = ! $lefttoright;
 +    $inner = render_node($node->firstChild, $lefttoright);
 +    $w = imagesx($inner)+6*UNIT;
 +    $h = imagesy($inner)+2*UNIT;
 +    $im = create_image($w, $h);
 +    imagecopy($im, $inner, 3*UNIT, 2*UNIT, 0,0, imagesx($inner), imagesy($inner));
 +    imageline($im,0,UNIT, $w, UNIT, $black);
 +    arrow($im, $w/2+UNIT/2, UNIT, $node->nodeName=='loop'?!$lefttoright:$lefttoright);
 +    arrow($im, 3*UNIT, 3*UNIT, $lefttoright);
 +    arrow($im, $w-2*UNIT, 3*UNIT, $lefttoright);
 +    imageline($im,UNIT,UNIT, UNIT, 3*UNIT, $black);
 +    imageline($im,UNIT,3*UNIT, 2*UNIT, 3*UNIT, $black);
 +    imageline($im,$w-UNIT,UNIT, $w-UNIT, 3*UNIT, $black);
 + imageline($im,$w-3*UNIT-1,3*UNIT, $w-UNIT, 3*UNIT, $black);
 +    return $im;
 +  } else if ($node->nodeName=='sequence') {
 +    $inner = render_childs($node, $lefttoright);
 +    if (!$lefttoright)
 +      $inner = array_reverse($inner);
 +    $w = count($inner)*UNIT-UNIT; $h = 0;
 +    for ($i = 0; $i<count($inner); $i++) {
 +      $w += imagesx($inner[$i]);
 +      $h = max($h, imagesy($inner[$i]));
 +    } $im = create_image($w, $h);
 +    imagecopy($im, $inner[0], 0, 0, 0,0, imagesx($inner[0]), imagesy($inner[0]));
 +    $x = imagesx($inner[0])+UNIT;
 +    for ($i = 1; $i<count($inner); $i++) {
 +      imageline($im, $x-UNIT-1, UNIT, $x, UNIT, $black);
 +      arrow($im, $x, UNIT, $lefttoright);
 +      imagecopy($im, $inner[$i], $x, 0, 0,0, imagesx($inner[$i]), imagesy($inner[$i]));
 +      $x += imagesx($inner[$i])+UNIT;
 +    } return $im;
 +  } else if ($node->nodeName=='choise') {
 +    $inner = render_childs($node, $lefttoright);
 +    $h = (count($inner)-1)*UNIT; $w = 0;
 +    for ($i = 0; $i<count($inner); $i++) {
 +      $h += imagesy($inner[$i]);
 +      $w = max($w, imagesx($inner[$i]));
 +    } $w += 6*UNIT; $im = create_image($w, $h); $y = 0;
 +    imageline($im, 0, UNIT, UNIT, UNIT, $black);
 +    imageline($im, $w-UNIT, UNIT, $w, UNIT, $black);
 +    for ($i = 0; $i<count($inner); $i++) {
 +      imageline($im, UNIT, $y+UNIT, $w-UNIT, $y+UNIT, $black);
 +      imagecopy($im, $inner[$i], 3*UNIT, $y, 0,0, imagesx($inner[$i]), imagesy($inner[$i]));
 +      arrow($im, 3*UNIT, $y+UNIT, $lefttoright);
 +      arrow($im, $w-2*UNIT, $y+UNIT, $lefttoright);
 +      $top = $y + UNIT;
 +      $y += imagesy($inner[$i])+UNIT;
 +    }
 +    imageline($im, UNIT, UNIT, UNIT, $top, $black);
 +    imageline($im, $w-UNIT, UNIT, $w-UNIT, $top, $black);
 +    return $im;
 +  } else if ($node->nodeName=='syntax') {
 +    $title = $node->getAttribute('title');
 +    $meta = $node->getAttribute('meta');
 +    $node = $node->firstChild;
 +    $names = array();
 +    $images = array();
 +    while ($node!=null) {
 +    $names[] = $node->getAttribute('name');
 +    $im = render_node($node->firstChild, $lefttoright);
 +    $images[] = $im;
 +       $node = $node->nextSibling;
 +    } $wn  = 0; $wr = 0; $h = 5*UNIT;
 +    for ($i = 0; $i<count($images); $i++) {
 +      $wn = max($wn, imagefontwidth(FONT)*strlen($names[$i]));
 +      $wr = max($wr, imagesx($images[$i]));
 +   $h += imagesy($images[$i])+2*UNIT;
 +    }
 +    if ($title=='') $h -= 2*UNIT;
 +    if ($meta=='') $h -= 2*UNIT;
 +    $w = max($wr+$wn+3*UNIT, imagefontwidth(1)*strlen($meta)+2*UNIT);
 +    $im = create_image($w, $h);
 +    $y = 2*UNIT;
 +    if ($title!='') {
 +      imagestring($im, FONT, UNIT, (2*UNIT-imagefontheight(FONT))/2,
 +      $title, $green);
 +      imageline($im, 0, 2*UNIT, $w, 2*UNIT, $green);
 +      $y += 2*UNIT;
 +    }
 +    for ($i = 0; $i<count($images); $i++) {
 +      imagestring($im, FONT, UNIT, $y-UNIT+(2*UNIT-imagefontheight(FONT))/2, $names[$i], $red);
 +      imagecopy($im, $images[$i], $wn+2*UNIT, $y, 0,0, imagesx($images[$i]) , imagesy($images[$i]));
 +      imageline($im, UNIT, $y+UNIT, $wn+2*UNIT, $y+UNIT, $black);
 +      imageline($im, $wn+2*UNIT+imagesx($images[$i])-1, $y+UNIT, $w-UNIT, $y+UNIT, $black);
 +      imageline($im, $w-UNIT, $y+UNIT/2, $w-UNIT ,$y+1.5*UNIT, $black);
 +      $y += 2*UNIT + imagesy($images[$i]);
 +    }
 +    imagestring($im, 1, UNIT, $h-2*UNIT+(2*UNIT-imagefontheight(1))/2,
 +      $meta, $silver);
 +    rr($im, 0,0,$w-1, $h-1, UNIT/2, $green);
 +    header('Content-Type: image/png');
 +    imagepng($im);
 +    return $im;
 +  }
 +}
 +
 +function render_childs($node, $lefttoright) {
 +   $childs = array();
 +   $node = $node->firstChild;
 +   while ($node!=null) {
 +     $childs[] = render_node($node, $lefttoright);
 +     $node = $node->nextSibling;
 +   } return $childs;
 +}
 +
 +function ebnf_scan(&$input) {
 +  global $ebnf_lexemes;
 +  $i = 0; $n = strlen($input); $m = count($ebnf_lexemes); $tokens = array();
 +  while ($i < $n) {
 +    $j = 0;
 +    while ($j < $m &&
 +      preg_match("/^{$ebnf_lexemes[$j]['expr']}/", substr($input,$i), $matches)==0) $j++;
 +    if ($j<$m) {
 +      if ($ebnf_lexemes[$j]['type']!=EBNF_WHITESPACE_TOKEN)
 +        $tokens[] = array('type' => $ebnf_lexemes[$j]['type'],
 +          'value' => $matches[0], 'pos' => $i);
 +      $i += strlen($matches[0]);
 + } else
 +   throw new Exception("Invalid token at position: $i");
 +  } return $tokens;
 +}
 +
 +
 +function ebnf_check_token($token, $type, $value) {
 +  return $token['type']==$type && $token['value']==$value;
 +}
 +
 +function ebnf_parse_syntax(&$tokens) {
 +  $dom = new DOMDocument();
 +  $syntax = $dom->createElement("syntax");
 +  $syntax->setAttribute('meta', META);
 +  $dom->appendChild($syntax);
 +  $i = 0; $token = $tokens[$i++];
 +  if ($token['type'] == EBNF_LITERAL_TOKEN) {
 +    $syntax->setAttribute('title',
 +      stripcslashes(substr($token['value'], 1, strlen($token['value'])-2 )));
 +    $token = $tokens[$i++];
 +  }
 +  if (!ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '{') )
 +    throw new Exception("Syntax must start with '{': {$token['pos']}");
 +  $token = $tokens[$i];
 +  while ($i < count($tokens) && $token['type'] == EBNF_IDENTIFIER_TOKEN) {
 +    $syntax->appendChild(ebnf_parse_production($dom, $tokens, $i));
 +    if ($i<count($tokens)) $token = $tokens[$i];
 +  } $i++; if (!ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '}'))
 +    throw new Exception("Syntax must end with '}': ".$tokens[count($tokens)-1]['pos']);
 +  if ($i<count($tokens)) {
 +    $token = $tokens[$i];
 +    if ($token['type'] == EBNF_LITERAL_TOKEN) {
 +      $syntax->setAttribute('meta',
 +        stripcslashes(substr($token['value'], 1, strlen($token['value'])-2 )));
 +    }
 +  }
 +    return $dom;
 +}
 +
 +function ebnf_parse_production(&$dom, &$tokens, &$i) {
 +  $token = $tokens[$i++];
 +  if ($token['type']!=EBNF_IDENTIFIER_TOKEN)
 +    throw new Exception("Production must start with an identifier'{': {$token['pos']}");
 +  $production = $dom->createElement("rule");
 +  $production->setAttribute('name', $token['value']);
 +  $token = $tokens[$i++];
 +  if (!ebnf_check_token($token, EBNF_OPERATOR_TOKEN, "="))
 +    throw new Exception("Identifier must be followed by '=': {$token['pos']}");
 +  $production->appendChild( ebnf_parse_expression($dom, $tokens, $i));
 +  $token = $tokens[$i++];
 +  if (!ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '.')
 +    && !ebnf_check_token($token, EBNF_OPERATOR_TOKEN, ';'))
 +    throw new Exception("Rule must end with '.' or ';' : {$token['pos']}");
 +  return $production;
 +}
 +
 +function ebnf_parse_expression(&$dom, &$tokens, &$i) {
 +  $choise = $dom->createElement("choise");
 +  $choise->appendChild(ebnf_parse_term($dom, $tokens, $i));
 +  $token=$tokens[$i]; $mul = false;
 +  while (ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '|')) {
 +    $i++;
 +    $choise->appendChild(ebnf_parse_term($dom, $tokens, $i));
 +    $token=$tokens[$i]; $mul = true;
 +  } return $mul ? $choise : $choise->removeChild($choise->firstChild);
 +}
 +
 +function ebnf_parse_term(&$dom, &$tokens, &$i) {
 +  $sequence = $dom->createElement("sequence");
 +  $factor = ebnf_parse_factor($dom, $tokens, $i);
 +  $sequence->appendChild($factor);
 +  $token=$tokens[$i]; $mul = false;
 +  while ($token['value']!='.' && $token['value']!='=' && $token['value']!='|'
 +    && $token['value']!=')' && $token['value']!=']' && $token['value']!='}') {
 +    $sequence->appendChild(ebnf_parse_factor($dom, $tokens, $i));
 +    $token=$tokens[$i]; $mul = true;
 +  } return $mul ? $sequence: $sequence->removeChild($sequence->firstChild);
 +}
 +
 +function ebnf_parse_factor(&$dom, &$tokens, &$i) {
 +  $token = $tokens[$i++];
 +  if ($token['type']==EBNF_IDENTIFIER_TOKEN) {
 +    $identifier = $dom->createElement("identifier");
 +    $identifier->setAttribute('value', $token['value']);
 +    return $identifier;
 +  } if ($token['type']==EBNF_LITERAL_TOKEN){
 +    $literal = $dom->createElement("terminal");
 +    $literal->setAttribute('value', stripcslashes(substr($token['value'], 1, strlen($token['value'])-2 )));
 +    return $literal;
 +  } if (ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '(')) {
 +    $expression = ebnf_parse_expression($dom, $tokens, $i);
 +    $token = $tokens[$i++];
 +    if (!ebnf_check_token($token, EBNF_OPERATOR_TOKEN, ')'))
 +      throw new Exception("Group must end with ')': {$token['pos']}");
 +    return $expression;
 +  } if (ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '[')) {
 +    $option = $dom->createElement("option");
 +    $option->appendChild(ebnf_parse_expression($dom, $tokens, $i));
 +    $token = $tokens[$i++];
 +    if (!ebnf_check_token($token, EBNF_OPERATOR_TOKEN, ']'))
 +      throw new Exception("Option must end with ']': {$token['pos']}");
 +    return $option;
 +  } if (ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '{')) {
 +    $loop = $dom->createElement("loop");
 +    $loop->appendChild(ebnf_parse_expression($dom, $tokens, $i));
 +    $token = $tokens[$i++];
 +    if (!ebnf_check_token($token, EBNF_OPERATOR_TOKEN, '}'))
 +      throw new Exception("Loop must end with '}': {$token['pos']}");
 +    return $loop;
 +  }
 +  throw new Exception("Factor expected: {$token['pos']}");
 +}
 +
 +?></code>
  
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