Lexers and parsers and processors, oh my!

Examples of domain specific languages…

Filter =3 or (>=7 and <12)

Expression language ((buyer.price * buyer.quantity)
 + (seller.price * seller.quantity))
 / (buyer.quantity + seller.quantity)

Templates

Hello {user.name | html}, you have {if (len(orders) == 1)} an order {else} {len(orders)} orders {end}

Data model definition employee { id int64 required; name string required; salary int32; address { street string; city string; country string; }; direct_reports list[employee]; status enum { working, on_leave, terminated }; }

Config rules http://mysite/ => { backends: 192.168.0.{101,102,103} } http://mysite/ [protocol=WebSocket] => { backends: 192.168.0.{201,202,203} } http://mysite/static/* => { dir: /home/website/staticfiles/ cache: 1day }

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 regular expression ain’t the answer

Use a parser generator • Write an input grammar (e.g. BNF) • Generates optimized code for your language of choice Tools like: flex, Bison, ANTLR, SableCC, peg…

Lexer Groups a stream of text into tokens number(100) minus paren_open word(price) plus number(5.5) paren_close star minus number(9.4) 100 - (price + 5.5) * -9.4

Lexer 100 - (price + 5.5) * -9.4 number(100) minus paren_open word(price) plus number(5.5) paren_close star minus number(9.4) // Grammar Tokens number = (['0' .. '9'])+ ('.' (['0' .. '9'])+)?; word = (['a' .. 'z'] | ['A' .. 'Z'] | ['0' .. '9'])+; minus = '-'; plus = '+'; star = '*'; slash = '/'; paren_open = '('; paren_close = ')'; whitespace = (' ' | '\t' | '\r'| '\n')+; Ignored Tokens whitespace;

Parser 100 - (price + 5.5) * -9.4 number(100) minus paren_open word(price) plus number(5.5) paren_close star SUBTRACT VALUE PARENS ADD VARIABLE VALUE MULTIPLY NEGATIVE number(9.4) VALUE minus Organizes token patterns into syntax tree

NEGATIVE number(9.4) VALUE Parser 100 - (price + 5.5) * -9.4 number(100) word(price) number(5.5) SUBTRACT VALUE PARENS ADD VARIABLE VALUE MULTIPLY Organizes token patterns into syntax tree

Productions additive = {add} additive plus multiplicative // a + ... | {subtract} additive minus multiplicative // a - ... | multiplicative // ... ; multiplicative = {multiply} multiplicative star term // a * ... | {divide} multiplicative slash term // a / .. | term // ... ; term = {variable} word // someVariable | {value} number // 22.0 | {negative} minus term // -22.0 | {parens} paren_open additive paren_close // ( ... ) ; number(100) minus paren_open word(price) plus number(5.5) paren_close star SUBTRACT VALUE PARENS ADD VARIABLE VALUE MULTIPLY NEGATIVE number(9.4) VALUE minus Parser

Syntax Tree abstract class Additive {
 …
 }
 class Add extends Additive { Additive getAdditive() { … } Multiplicative getMultiplicative() { … } …
 }
 class Subtract extends Additive { Additive getAdditive() { … } Multiplicative getMultiplicative() { … } } // e.g. SableCC generated Java class heirachy…
 abstract class Term {
 …
 }
 class Variable { Word getWord() { … } … }
 class Value extends Term { Number getNumber() { … }
 … }
 class Negative extends Term { Term getTerm() { … }
 … } number(100) word(price) number(5.5) SUBTRACT VALUE PARENS ADD VARIABLE VALUE MULTIPLY NEGATIVE number(9.4) VALUE Auto generated from grammar

Joe recommends... C Peg, Flex+Bison, ANTLR3, Ragel C++ SableCC, ANTLR4, Boost Spirit, (and C ones) C# ANTLR4 Java SableCC, ANTLR4, JFlex+BYACC/J, Ragel Python ANTLR4 Ruby Treetop, Ragel JavaScript Jison, PEG.js, ANTLR4 Go Ragel

EOF @joewalnes