| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next

C[omp]ute

Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

C-ORM: docs, API.

Last 100 entries

Why Information Grows; The Blindness Of The Chilean Elite; Some Victoriagate Links; This Is Why I Left StackOverflow; New TLS Implementation; Maths for Physicists; How I Am 8; 1000 Word Philosophy; Cyberpunk Reading List; Detailed Discussion of Message Dispatch in ParserCombinator Library for Julia; FizzBuzz in Julia w Dependent Types; kokko - Design Shop in Osaka; Summary of Greece, Currently; LLVM and GPUs; See Also; Schoolgirl Groyps (Maths); Japanese Lit; Another Example - Modular Arithmetic; Music from United; Read Agatha Christie for the Plot; A Constructive Look at TempleOS; Music Thread w Many Recommendations; Fixed Version; A Useful Julia Macro To Define Equality And Hash; k3b cdrom access, OpenSuse 13.1; Week 2; From outside, the UK looks less than stellar; Huge Fonts in VirtualBox; Keen - Complex Emergencies; The Fallen of World War II; Some Spanish Fiction; Calling C From Fortran 95; Bjork DJ Set; Z3 Example With Python; Week 1; Useful Guide To Starting With IJulia; UK Election + Media; Review: Reinventing Organizations; Inline Assembly With Julia / LLVM; Against the definition of types; Dumb Crypto Paper; The Search For Quasi-Periodicity...; Is There An Alternative To Processing?; CARDIAC (CARDboard Illustrative Aid to Computation); The Bolivian Case Against Chile At The Hague; Clear, Cogent Economic Arguments For Immigration; A Program To Say If I Am Working; Decent Cards For Ill People; New Photo; Luksic And Barrick Gold; President Bachelet's Speech; Baltimore Primer; libxml2 Parsing Stream; configure.ac Recipe For Library Path; The Davalos Affair For Idiots; Not The Onion: Google Fireside Chat w Kissinger; Bicycle Wheels, Inertia, and Energy; Another Tax Fraud; Google's Borg; A Verion That Redirects To Local HTTP Server; Spanish Accents For Idiots; Aluminium Cans; Advice on Spray Painting; Female View of Online Chat From a Male; UX Reading List; S4 Subgroups - Geometric Interpretation; Fucking Email; The SQM Affair For Idiots; Using Kolmogorov Complexity; Oblique Strategies in bash; Curses Tools; Markov Chain Monte Carlo Without all the Bullshit; Email Para Matias Godoy Mercado; The Penta Affair For Idiots; Example Code To Create numpy Array in C; Good Article on Bias in Graphic Design (NYTimes); Do You Backup github?; Data Mining Books; SimpleDateFormat should be synchronized; British Words; Chinese Govt Intercepts External Web To DDOS github; Numbering Permutations; Teenage Engineering - Low Price Synths; GCHQ Can Do Whatever It Wants; Dublinesque; A Cryptographic SAT Solver; Security Challenges; Word Lists for Crosswords; 3D Printing and Speaker Design; Searchable Snowden Archive; XCode Backdoored; Derived Apps Have Malware (CIA); Rowhammer - Hacking Software Via Hardware (DRAM) Bugs; Immutable SQL Database (Kinda); Tor GPS Tracker; That PyCon Dongle Mess...; ASCII Fluid Dynamics; Brandalism; Table of Shifter, Cassette and Derailleur Compatability; Lenovo Demonstrates How Bad HTTPS Is; Telegraph Owned by HSBC; Smaptop - Sunrise (Music)

© 2006-2015 Andrew Cooke (site) / post authors (content).

Implementing a Regular Expression Engine

From: "andrew cooke" <andrew@...>

Date: Sun, 22 Mar 2009 20:21:29 -0400 (CLT)

A week or two ago I posted a message here saying I had implemented a
regular expression engine.  I deleted it today...

It would be more correct to say I have been learning about how to
implement a regular expression engine by failing to implement one. 
However, I am making some progress.

My initial take on regular expressions was to treat them as a tree
(basically the parse tree you would expect from their string
representation).  While that wasn't necessarily wrong, it made life a lot
harder than it should have been - it is much easier to model them as
directed graphs.

One translated into graph form, it's pretty easy to generate a
non-deterministic finite automaton that does the appropriate matching. 
That's because it's pretty much the same as the core loop for trampolined
recursive descent parser.  You can even get all the different matches with
back-tracking (technically I am therefore implementing the NFA (with eta
transitions, which are useful to order different parts of the automoaton)
using a PDA, which is more general and so explains the connection to
recursive descent parsing).

That's as far as I have got.  A previous post here gives references on how
to translate the NFA to a DFA (which being deterministic is a lot easier
to "run"; the flip side is that it may have exponentially more states and
there's no way to implement it so that all variations (non greedy matching
etc) are returned).

My aim is to make this NFA a standard LEPL matcher.  The DFA
implementation will be used for lexing.  In the future I'd also like to
see if compiling Any, Literal, Word etc (and combinations with And, Or,
Repeat) to NFA make things faster.

Andrew

http://en.wikipedia.org/wiki/Automata_theory - the table at the bottom is
particularly useful.

Initial DFA Results

From: "andrew cooke" <andrew@...>

Date: Tue, 24 Mar 2009 20:52:27 -0400 (CLT)

This is sweet - it's nice when a piece of code does things you didn't
really expect it to do (I mean, in theory, I can see why it works, but in
practice, the way it solves the problems is almost "intelligent").

Here's an example for the regexp a(bc|b*d):

     a      d
  0 ---> 1 +--> 2* <--+----.
           |b     c/d |    |
           `--> 3 +---'    |
                  |b      d|
                  `--> 4 +-'
                  '      |b
                  |      |
                  `------'

or, as printed (the [...] are the original nfa nodes):

  0 [0]       a:1,
  1 [3, 5, 6] d:2;b:3,
  2 [1, 2]    label,
  3 [4, 5, 6] b:4;[c-d]:2,
  4 [5, 6]    b:4;d:2

Andrew

Original NFA

From: "andrew cooke" <andrew@...>

Date: Tue, 24 Mar 2009 21:11:09 -0400 (CLT)

Just for fun, here's the underlying NFA (with eta/empty transitions). 
This is easier to construct (both by hand and in code)

      a     b      c
  0 ---> 3 +--> 4 ----------+> 1 ---> 2 "label"
           |                |
           |              d |
           `--> 5 +--> 6 ---'
           '      |
           |   b  |
           `------'

0 a:3
1 2
2 label
3 b:4 5
4 c:1
5 b:5 6,
6 d:1

(This is all left to right except for the b* loop from/to 5 at the bottom)

Andrew

Epsilon!

From: "andrew cooke" <andrew@...>

Date: Tue, 24 Mar 2009 21:25:35 -0400 (CLT)

I've been calling my epsilons etas!  Ooops.  Andrew

Comment on this post