Regular Expression Engine in C# (the Story)

A “long time ago”, more precisely 3 years ago, I was studying Automata and Formal Languages which was a Computer Engineering discipline in the 6th semester out of 10 at UBM.

At that time I was amazed by the new things I was learning such as NFA - Nondetermistic Finite Automaton, DFA - Deterministic Finite Automaton, Finite State Machine and Regular Expressions. For more on this, read my last post: Fortran Numerical Constants Recognizer.

For the sake of my development I started searching for programming related material that could put me informed about such amazing computer science constructs.

I then came across a series of articles at called Algorithmic Forays. It’s been written by Eli Bendersky. This guy did a great job putting together the base for a Regular Expression Engine that exercise the concepts of NFA and DFA. His code was presented in C++ and I took the time to learn a bit more about this powerful language too.

After reading Algorithmic Forays from part 1 through part 6 I started thinking about translating the code from C++ to C#. That’d be a great opportunity to grasp new C# constructs and at the same time get in touch with material related to the discipline afore mentioned.

Setting that in my mind I put my hands at work and after struggling here and there I got the code translated. I also thought about writing a Portuguese version of the article to be publicized at my university’s scientific magazine. The idea was to write an adapted version of the article presenting both code (C++ and C#) showing how to achieve the same results using different programming languages data structures.

I’ve sent a message to Eli asking him permission to write a Portuguese version of the article. You can read the e-mail I’ve sent Eli and his reply below:

Leniel Macaferi <le@…>

Algorithmic Forays for a Regex Parser

le@... <le@...>                                                              Wed, Jan 3, 2007 at 5:43 AM

To: eli…@…

Cc: Wellington Magalhães Leite <wml@…>

Dear Eli,

My name is Leniel and I'm a student of Computer Engineering here in
Brazil. I'm currently in the 9th term out of 10.
You can see my basic profile at

Today I'm on my vacation and I'm going to come back studying on February.

I'm passionate for anything related to technology and specially
programming which is my hobby just like yours.

Firstly I'd like to congratulate you by your advanced knowledge of
such great topics about Computer Science and Computer Engineering like
the one you discussed in the article Algorithmic Forays - Part 1 to 6.

Well, I've read your biography at
and I'm currently seeing your photo album. You're a great example for
us to follow due your characteristics as liking to study and liking to
travel the whole world. It's just what I want to do too as a
professional and as a traveler! You really do very well on this.

Your site is all of good! Think about the entire internet filled with
sites just like yours. With a complete bunch of interesting
information! My God it's a dream.

Coming to the real reason I'm writing to you - it's about a project
that has been proposed to me and a friend of mine called Wellington.
When we were in the 7th term at the university during our class of
Laboratory of Compiler Construction studying Lex and Yacc, our teacher
asked us to prepare an article about the process of creation of an
engine to parse Regular Expressions. Since that moment we started
searching the Internet for something that we could use as a base for
our article. For our lucky, we found your article Algorithmic Forays
at that is amazing for everyone to understand. It's
simple and direct with implementation examples.
During the 6th term at the university we had a discipline called
Automatons and Formal Languages. Was at this period that we learned
topics regarding Finite State Machines (FSM), Deterministic Finite
Automatons (DFA), Nondeterministic Finite Automatons (NFA) and Regular
Expressions. So, from there we know that a DFA + NFA = FSM! We've also
implemented some code for covering and getting a better understanding
of how such topics work in reality.

In true, we started our project preparing an article on April, May of
2006, when I particularly started converting all your Algorithmic
Foray's C++ code to the C# language that is the one I most like to use
for writing any kind of code. But due our other tasks, in other words,
other disciplines, we stopped working on this project. It's important
to mention that I could finalize translating your C++ code to C# on
the middle of May 2006.

I've learned such great things working with the code as mastering my
knowledge with the Generics in C#. I even got one third party DLL
called C5.dll from which includes lots
of new classes to work with sets of data as is necessary when dealing
with regular expressions.

Yesterday, 1/2, when I woke up I just thought why not to continue
working on that project about a regex parser. So, I started again with
new motivations.

I started translating your whole article from English to Portuguese
and I'd like to ask you if we could use some text and diagrams of your
article as the base of our article and we would present it with both
versions of code, C++ and C#.

Our aim is to forward it to be issued in our university's scientific
magazine and propagate to the ones interested – see
(it's in Portuguese - you're learning Spanish and it's a little bit
alike. I've already publicized one article in this magazine with the
title: Development and Numeric Simulation of Algorithms for
Computational Resolution of Ordinary Differential Equations. It was a
project I developed while I was in the 4th term. The code was
developed using C++. Further, this article will be available in the
Internet through an indexing project that's been prepared by a
university here in Brazil.

We'll certainly cite the article's fonts as you can see in the
preliminary code I'm going to send you in this e-mail. Your great
initiative of publicizing the article must be preserved by any mean.
Haven't you used any other bibliography? Was it developed by just your

Remember that this code I'm sending you needs a review because I have
to pass through it to get it all in mind again and see if everything
is Ok. To run the code you just need to recompile it using a C#
compiler. I use the Microsoft Visual Studio 2005 Express Edition as
you mentioned in your home page here
The initial parameter I'm using is set in the command line argument
inside the project's properties in Debug and is as follow:
RegularExpressionEngine "(a|b)*abb" aaababb
It's seems to be working Ok.
Try it for yourself!

From now on we're just waiting for your reply.

By the way, if you and your wife want to come to visit Brazil you can
certainly come to my house. I'd like to have you as my guests. Who
knows, one day I can go to Israel and meet you personally. That would
be great too. Israel is the apple of God's eye!

I simply think we have the same liking!

It was a pleasure to write to you.

My best regards,
                         Leniel Braz de Oliveira Macaferi
                         Volta Redonda, Rio de Janeiro, Brazil

Leniel Macaferi <le@…>

Algorithmic Forays for a Regex Parser

Eli Bendersky <eli@…>                                                Sun, Jan 21, 2007 at 2:03 AM

To: "le@…" <le@…>

Hello Leniel,

First of all, thanks for all the compliments - I'm flattered.
You can certainly use any part of the articles and the diagrams for your translation. As for my sources, I certainly didn't invent anything - I took all the material from the "Dragon Book" ("Compilers" by Aho, Sethi and Ullman), perhaps with the help of some googling, but as far as I remember, the book was the main source.

Keep being enthusiastic about your studies. That's a good thing :-)



As you can see from the above e-mail, Eli agreed and so it was OK to write a Portuguese version of his articles.

As the semesters passed and new commitments queued I stopped translating the articles and I have just decided this past week that now it’s a good time to come back to it and finish the planned objective of 2 years ago.

While I write the next blog posts I’ll translate the remaining articles to Portuguese. I expect to publish it online here on this blog so that it can help people out there to learn this fascinating subject of Computation Theory.

I’ll follow a different path to explain how the regular expression engine work, that is, from part 1 to part 6 I’ll be showing each division of the engine by means of its representing class in code. I’ll do so, so that I can present each part of the translated code (C++ to C#) in such a way that it gets easier to understand. I’ll use a top down approach, that is, I’ll begin showing the higher level class and then I’ll go deep to its dependencies.

This is the story behind the Regular Expression Engine in C# I’ll be presenting in the next posts, starting with this one.

Hope that you like it as much as I do. This was for sure one of or even the best discipline I’ve had in the Computer Engineering course and one of the more exciting things to write about.

Updated on 5/12/2009 09:41:00 PM

As I finished writing the series of posts, here goes the list that points to them:

Regex engine in C# - the Regex Parser
Regex engine in C# - the NFA
Regex engine in C# - the DFA
Regex engine in C# - matching strings

Source code: