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Compiler Construction Tools

Tools.ppt

CS 4623

Compiler Construction

CS4173 Compiler Construction – 2 –

Introduction

 The process of writing a lexical analyzer and parser can be automated by using tools

 Compiler construction tools are also known as compiler compilers or compiler generators

 Usually scanner generators accept token definitions as regular expressions, while parser generators accept grammar in BNF (or some variation such as EBNF)

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Downloading the Tools

 Windows versions of Lex and Yacc can be downloaded from:

http://www.bumblebeesoftware.com/downloads.htm

 JFlex can be downloaded from:

http://www.jflex.de/download.html

 COCO/R can be downloaded from:

http://cs.ru.ac.za/homes/cspt/cocor.htm

CS4173 Compiler Construction – 5 –

 JFlex takes as input a script that uses regular expressions to describe a set of lexeme patterns

 It generates a Java class implementing a scanner that recognizes those patterns

 An action is associated with each pattern, which is a chunk of Java code – each time the scanner recognizes a pattern, the associated action is executed

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 User Code section:

The user code section contains code that appears outside of the scanner class

Typically, it contains import or package declarations, but can include an additional class if you like.

The code in this section will appear above the generated scanner class in the .java file generated by JFlex

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 Options and Declarations section contains three things:  Specification of various options, such as the name of the scanner class, the return type and name of its "next token" method, etc.  Additional code that you want to place into the scanner class. For example, if you need to add fields or helper methods into the scanner class, this is where you put them.  Macro definitions that you want to use in your regular expressions.

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 Lexical Rules section:

This section contains token definitions and actions that should be executed whenever a matching lexeme is found

Most of the time, the action would contain a return statement which will return a value from the scanner method.

In case of patterns that define text that should be ignored, such as whitespace and comments, the action may be empty

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 Example of Lexical Rules section: [0-9]+ { return new Token(Token.INTCONST, yytext()); }

[a-zA-Z][a-zA-Z0-9] { return new Token(Token.IDENTIFIER, yytext()); }*

"for" { return new Token(Token.KWD_FOR, yytext()); }

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 Rules to construct regular expressions in JFlex (contd…):  The Kleene closure (or "star") operation is specified by the * character, e.g. the expression [a-z][a-z]* specifies strings of lowercase letters with at least one letter in them.  As a shorthand for "one or more", the + operator can be used. The expression [a-z]+ is equivalent to [a-z][a-z].  As a shorthand for "zero or one", the? operator can be used. The expression [a-z][a-z]? is equivalent to [a-z] | [a-z][a-z].  Macro names are written in curly braces. So, the expression {Whitespace}+ would match one or more characters of whitespace, assuming an appropriately defined macro called Whitespace.  The. character (appearing outside of a string) matches any character in the character set except a newline. So, the expression a. matches all strings that begin with a lowercase "a".