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Parsing Expression Grammars. Aaron Hoffer CSS 548 Autumn 2012. PEGs. What if Flex and Yacc were one program? What if you could use the same regular expression patterns as Flex in your parser generator? What if Yacc supported… ! (not “XYZ…”) * (zero or more).
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Parsing Expression Grammars Aaron Hoffer CSS 548 Autumn 2012
PEGs • What if Flex and Yacc were one program? • What if you could use the same regular expression patterns as Flex in your parser generator? • What if Yacc supported… • ! (not “XYZ…”) • * (zero or more)
/* Scanning C comments with Flex */ <INITIAL>”/*” { BEGIN(IN_COMMENT);} <IN_COMMENT>[^*]*\*+ { BEGIN(WARNING);} <WARNING>[^/] { BEGIN(IN_COMMENT);} <WARNING>”/“ { BEGIN(INITIAL); }
/* Scanning C comments with PEG */ Comment: ”/*” (“*” !”/” / [^*])* “*/” Let’s break it into multiple rules to see what it means: Comment: ”/*” Middle “*/” Middle: (Asterisk | NotAsterisk)* Asterisk: “*” !”/” NotAsterisk: [^*]
/*Nested /*comments*/ with PEG*/ • Add the non-terminal Comment into Middle • Now parses nested comments Comment:”/*” Middle “*/” Middle: (Comment| Asterisk | NotAsterisk)* Asterisk: “*” !”/” NotAsterisk: [^*]
What is a PEG? • Not context-free grammar or regular expression • Are not ambiguous. PEG parsers matches rules in the order they are defined • Are a formal description of what a recursive descent parser with back-tracking is capable of parsing • Support predicates like “not” and “and” because the parser can look ahead and then back-track
Domain Specific Languages and PEGs • Alan Kay Viewpoints Research Institute – lots of research on PEGs • Their vision: tiny DSLs cooperating in the same environment to accomplish big tasks (sounds like Lisp to some critics) • VPRI’s STEP project demonstrated an OS, graphics system, word processor, spreadsheet, etc. in 20 KLOCs • How VPRI did it? • Throw everything away. • Create self-hosting language and little DSLs. • Collapse code size by factor of 1000