The document introduces building a parser in PHP by explaining reasons for common fears around parsing, showing examples of language grammars like BNF and EBNF, and demonstrating how to generate a parser in PHP using PEG parsing expressions to parse a sample query language across multiple versions, with the potential to optimize parsed queries.

1. Let’s build a Parser!
A short introduction to parsing with PHP
Boy Baukema
June 9th 2012, Amsterdam

2. 2
Source: http://www.sxc.hu/photo/1384894

3. Boy Baukema
Software Engineer @ Ibuildings
3

4. Reasons for common
fear of writing parsers:
1. Never took
compiler class, think it
is scary.
2. Did take compiler
- Martin Fowler
4

5. Language cacaphony
5
Source: http://www.wordle.net/show/wrdl/5292561/
Languages_used_in_PHP_Web_Development

6. Lookahead (?=
Languages
Parsing
QueryLang
Parsing PHP code
Resources
6

7. RegExes
And now you have two problems...
7

8. Mail::RFC822::Address
(?:(?:rn)?[ t])*(?:(?:(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[ t]
)+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[ t]))*"(?:(?:
rn)?[ t])*)(?:.(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031]+(?:(?:(
?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[
t]))*"(?:(?:rn)?[ t])*))*@(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-0
31]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*
](?:(?:rn)?[ t])*)(?:.(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031]+
(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:
(?:rn)?[ t])*))*|(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[ t])+|Z
|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[ t]))*"(?:(?:rn)
?[ t])*)*<(?:(?:rn)?[ t])*(?:@(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:
rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[
t])*)(?:.(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)
?[ t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[ t]
)*))*(?:,@(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[
t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[ t])*
)(?:.(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[ t]
)+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?:rn)?[ t])*))*)
*:(?:(?:rn)?[ t])*)?(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[ t])+
|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[ t]))*"(?:(?:r
n)?[ t])*)(?:.(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:
rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?[ t
]))*"(?:(?:rn)?[ t])*))*@(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031
]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](
?:(?:rn)?[ t])*)(?:.(?:(?:rn)?[ t])*(?:[^()<>@,;:".[] 000-031]+(?
:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|[([^[]r]|.)*](?:(?
:rn)?[ t])*))*>(?:(?:rn)?[ t])*)|(?:[^()<>@,;:".[] 000-031]+(?:(?
:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(?:(?:rn)?
[ t]))*"(?:(?:rn)?[ t])*)*:(?:(?:rn)?[ t])*(?:(?:(?:[^()<>@,;:".[]
000-031]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|
.|(?:(?:rn)?[ t]))*"(?:(?:rn)?[ t])*)(?:.(?:(?:rn)?[ t])*(?:[^()<>
@,;:".[] 000-031]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|"
(?:[^"r]|.|(?:(?:rn)?[ t]))*"(?:(?:rn)?[ t])*))*@(?:(?:rn)?[ t]
)*(?:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:
".[]]))|[([^[]r]|.)*](?:(?:rn)?[ t])*)(?:.(?:(?:rn)?[ t])*(?
:[^()<>@,;:".[] 000-031]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[
]]))|[([^[]r]|.)*](?:(?:rn)?[ t])*))*|(?:[^()<>@,;:".[] 000-
031]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|"(?:[^"r]|.|(
?:(?:rn)?[ t]))*"(?:(?:rn)?[ t])*)*<(?:(?:rn)?[ t])*(?:@(?:[^()<>@,;
:".[] 000-031]+(?:(?:(?:rn)?[ t])+|Z|(?=[["()<>@,;:".[]]))|[([
8
Source: http://www.ex-parrot.com/pdw/Mail-RFC822-Address.html

9. Choamsky hierarchy
9
Source: http://en.wikipedia.org/wiki/File:Chomsky-hierarchy.svg

10. HTTP 1.1 Accept Header BNF
Accept = "Accept" ":"
#( media-range [ accept-params ] )
media-range = ( "*/*"
| ( type "/" "*" )
| ( type "/" subtype )
) *( ";" parameter )
accept-params = ";" "q" "=" qvalue
*( accept-extension )
accept-extension = ";" token
[ "=" ( token | quoted-string ) ]
10
Source: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

11. Arithmetic expression BNF
<expression> ::= <term>
| <expression> "+" <term>
<term> ::= <factor>
| <term> "*" <factor>
<factor> ::= <constant>
| <variable>
| "(" <expression> ")"
<variable> ::= "x" | "y" | "z"
<constant> ::= <digit>
| <digit> <constant>
<digit> ::= "0" | "1" | "2" | "3"
| "4" | "5" | "6" | "7"
| "8" | "9"
11
Source: http://en.wikipedia.org/wiki/Syntax_diagram

12. Recursion in BNF
Production
<constant> ::= <digit>
| <digit> <constant>
<digit> ::= "0" | "1" | "2" | "3"
Terminal | "4" | "5" | "6" | "7"
| "8" | "9"
12

13. Matching 123
<constant>
1 <digit>
<constant>
2 <digit>
<constant>
3 <digit>
<constant> ::= <digit>
13
Source: https://secure.flickr.com/photos/threedots/110586879/

14. Arithmetic expression EBNF
expression = term , {"+" , term};
term = factor , {"*" , factor};
factor = constant
| variable
| "(" , expression , ")";
variable = "x"
| "y"
| "z";
constant = digit , {digit};
digit = "0" | "1" | "2" | "3"
| "4" | "5" | "6" | "7"
| "8" | "9";
14
Source: http://en.wikipedia.org/wiki/Syntax_diagram

15. Parsing Expression Grammar
expression = term ("+" term)*
product = factor ("*" factor)*
factor = constant
/ variable
/ "(" expression ")"
variable = "x"
/ "y"
/ "z"
constant = [0-9]+
15
Source: https://secure.flickr.com/photos/sasastro/5590210866/

16. So how will this help
me parse a language?
16

17. Parser Generators for PHP
Lime-php
LALR(1) , 2008, abandoned
PHP_ParserGenerator
LALR(1), 2010, abandoned
Loco
combinatory parsing, 2011,
alpha
php-peg
PEG, 2012, active?, alpha
17

18. QueryLang
https://github.com/relaxnow/QueryLang
18

19. QueryLang: Example query
parsers OR 123 AND (dpc OR phpbnl)
Query (OR)
|-- Term - "parsers"
|-- Query (AND)
|-- Term - "123"
|-- Query (OR)
|-- Term - "dpc"
|-- Term - "phpbnl"
19

20. v1/Peg/grammar.peg.inc
/*!* QueryLangV1
Term: /[wd]+/
*/
public function parse()
{
$match = $this->match_Term();
if (!$match) {
return '';
}
return $match['text'];
}
20

21. v1/Peg/Parser.php - generated match_Term
/* Term: /[wd]+/ */
protected $match_Term_typestack =
array('Term');
function match_Term ($stack = array()) {
$matchrule = "Term"; $result = $this-
>construct($matchrule, $matchrule, null);
if (( $subres = $this->rx( '/[wd]+/' ) ) !==
FALSE) {
$result["text"] .= $subres;
return $this->finalise($result);
}
else { return FALSE; }
}
21

22. v1/Peg/grammar.peg.inc test
$parser = new Parser('test');
print_r($parser->parse());
// test
$parser = new Parser('test 123');
print_r($parser->parse());
// test
22

23. v2/Peg/grammar.peg.inc
/*!* QueryLangV2
Query: Term (> Term)*
Term: /[wd]+/
*/
public function parse()
{
$result = $this->match_Query();
return $result['query'];
}
23

24. v2/Peg/grammar.peg.inc (cont.)
public function Query__construct(&$result)
{
$result['query'] = new NodeQuery();
}
public function Query_Term(&$result, $sub)
{
$term = new NodeTerm($sub['text']);
$result['query']->addTerm($term);
}
24

25. v2/Peg/grammar.peg.inc test
$parser = new Parser('test 123');
print_r($parser->parse());
Query
|-- Term - "test"
|-- Term - "123"
25

26. v3/Peg/grammar.peg.inc
/*!* QueryLangV3
Query: AndQuery ([ "OR" ] AndQuery)*
AndQuery: Term ([ "AND" ] Term)*
Term: "(" Query ")" | Value:/[wd]+/
*/
public function parse()
{
$node = $this->match_Query();
return $node['query'];
}
26

27. v3/Peg/grammar.peg.inc (cont.)
Query: AndQuery ([ "OR" ] AndQuery)*
AndQuery: Term ([ "AND" ] Term)*
public function Query__construct(&$r) {
$r['query'] = new NodeQuery('OR');
}
public function Query_AndQuery(&$r, $s) {
$r['query']->add($s['query']);
}
public function AndQuery__construct(&$r) {
$r['query'] = new NodeQuery('AND');
}
public function AndQuery_Term(&$r, $s) {
$r['query']->add($s['query']);
27
}

28. v3/Peg/grammar.peg.inc (cont.)
/*!* QueryLangV3
Term: "(" Query ")" | Value:/[wd]+/
*/
public function Term_Query(&$r, $s){
$r['query'] = $s['query'];
}
public function Term_Value(&$r, $s){
$r['query']= new NodeTerm($s['text']);
}
28

29. v3/Peg/grammar.peg.inc test
$parser = new Parser('a AND b OR c');
Query (OR)
|-- Query (AND)
| |-- Term - "a"
| |-- Term - "b"
|-- Query (AND)
|-- Term - "c"
29

30. Optional: Optimizer / Semantic checking
30

31. Optimized query
$parser = new Parser('a AND b OR c');
$query = $parser->parse();
$queryOptimizer = new Optimizer($query);
$query = $queryOptimizer->optimize();
Query (OR)
|-- Term - "c"
|-- Query (AND)
| |-- Term - "a"
| |-- Term - "b"
31

32. Manual Parser Building: Predictive parsing
32
Source: http://en.wikipedia.org/wiki/File:PsychicBoston.jpg

33. Manual Parser Building: Lexing
Characters get turned into tokens by a lexical
analyzer. Also called lexer, scanner or
tokenizer.
"a OR (b)"
'term' => "a"
'OR'
'LeftParen'
'term' => "b"
'RightParen' 33

34. Manual Parser Building: Lexing
if ($this->_match('LeftParen', '/^(()/')) {continue;}
if ($this->_match('RightParen', '/^())/')) {continue;}
if ($this->_match('OR', '/^(OR)/i'))
{continue;}
if ($this->_match('AND', '/^(AND)/i')) {continue;}
if ($this->_match('TermValue', '/^([wd]+)/i'))
{continue;}
if ($this->_match('WS', '/^s+/', true)) {continue;}
34

35. Manual Parser Building: Lexing - UML
35
Source: http://commons.wikimedia.org/wiki/File:Willem-
Alexander,_Prince_of_Orange.jpg

36. Manual Parser Building: Parsing
Non-terminals become methods
protected function _query();
protected function _andQuery();
protected function _term();
Parse to a tree structure.
36

37. Manual Parser Building: Parsing - UML
37

38. Manual Parser Building: example non-terminal
protected function _query() {
$query = new NodeQuery('OR');
$leftTerm = $this->_andQuery();
$query->add($leftTerm);
while($this->_tokenStream->look()->getType()
=== 'OR') {
$this->_tokenStream->expect('OR');
$rightTerm = $this->_andQuery();
$query->add($rightTerm);
}
return $query;
} 38

39. Predictive Parsing: Warning!
Tokens must be decidable with a fixed lookahead
<term> ::= <TermValue> "-" <TermValue>
| <TermValue>
| "(" <Query> ")"
No left recursion
<orQuery> ::= <orQuery> ("OR" <orQuery>)?
| <term>
39

40. But I wanna parse
40

41. PHP Parsers
PHP_Depend
1.0.0
PHP 5.4
PHP-Parser
alpha
PHP 5.4
phc
0.3.0.1 (unmaintained)
PHP 5.2 (?)
41

42. PHPDepend Abstract Syntax Tree example
$string = "Manuel $Pichler <{$email}>";
PHP_Depend_Code_ASTString
|-- ASTLiteral - "Manuel "
|-- ASTVariable - $Pichler
|-- ASTLiteral - " <"
|-- ASTCompoundExpression - {...}
| |-- ASTVariable - $email
|-- ASTLiteral - ">"
42

43. Resources
43

44. More resources
Examples of modern parsers in PHP:
Twig (Predictive Parser)
Behats Gherkin (Predictive Parser)
Smarty 3 (LALR parser)
More information:
Rich Programmer Food by Steve Yegge
Let’s Build a Compiler, by Jack Crenshaw
nathansuniversity.com
Coursera: Compilers by Stanford University
SE-Radio: Episode 182: DSLs
44

45. QUESTIONS?
Joind.in: https://joind.in/6257
Twitter: @relaxnow
E-mail: boy@ibuildings.nl
Slideshare: http://slidesha.re/INY43R 45
GitHub: https://github.com/relaxnow/QueryLang