You usually can use them unchanged. See section porting from JLex of the manual for more information on that topic. Check out the JFlex website. There is an announcement mailing list for announcements for new JFlex releases. You can. You can use your generated code without restriction.

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You have available to you a sample lexer built with JLex. In the great tradition of building complex systems, I suggest you start building your own lexer by incremental modification of this one.

Do it incrementally - one change at a time! There are two files provided one called sym. There is also the file sample. Before you start working with JLex, you need to set some things up. Firstly, you need to make sure JLex is available to you. The JLex system needs to appear within a directory called wait for it! If you are using the JLex system on the Department's computers, this should already be done for you.

If you are using JLex at home, you'll need to set it up. You will need to create a directory called "compiler" and copy the sym. Compile that file in the compiler directory it is in a Java package called compiler. You are now ready to run Jlex. Move back to the directory that contains the sample. Running Jlex is a breeze. You just feed in the input file sample. Like this:. The result of running Jlex on sample.

We now need to run that through the Java compiler before we can run the program! If you get errors at this stage, you need to go back and edit the. What you now have is the sample class compiled and ready to go. Shown below is a sample run. Make sure you use a capital "S" in sample! As you were told in lectures, JLex generates a Java class called yylex. The constructor method for this class takes either a java. Reader or java. You will note the start of the main program contained in the sample file:.

The JLex manual is available here. The manual assumes you understand what you are doing! It is only a reference! You should not need to do this.

However you may want to install it at home. There are two ways of installing JLex - the easy and the not as easy but not that hard either way! Download the jlex. Simply type:. Copy the file Main. Create a directory called "JLex" somewhere. I created a JLex subdirectory in there. Compile the JLex system by:. If you ignored the advice above about obtaining JLex from the Department's JLex directory and instead forged off on your own and downloaded JLex from the web - you may now be sitting looking at a bunch of Java warnings that will recurr each time you use JLex.

Copy our version. Like we said. Life is too short! You should now have JLex installed, as it were. Make sure it doesn't point to JLex! How to use JLex. Running Jlex. JLex interfacing. JLex Manual. Installing your own JLex. Processing second section -- JLex declarations. Processing third section -- lexical rules. Creating NFA machine representation. NFA comprised of states.

Working on character classes. NFA has 43 distinct character classes. Creating DFA transition table. Working on DFA states Minimizing DFA transition table.

Outputting lexical analyzer code.


Table of Contents

In this lecture we will look LR parsers and parser generators. The slides for this lecture can be found here and here. Pratt and Zelkowitz, chapter 4. Gagnon and Laurie J. The JLex manual, by Elliot Berk. The manual can be downloaded from.


Programming Languages and Compilers

JFlex is a lexical analyser generator for Java written in Java. A lexical analyser generator takes as input a specification with a set of regular expressions and corresponding actions. It generates a program a lexer that reads input, matches the input against the regular expressions in the spec file, and runs the corresponding action if a regular expression matched. Lexers usually are the first front-end step in compilers, matching keywords, comments, operators, etc, and generating an input token stream for parsers.

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