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Sameer Borate:
Cron Expression Parser in PHP
Jul 21, 2015 @ 10:15:09

If you've ever worked with the "cron" tool on a unix-based system, you know that there's a special syntax that comes along with defining when the commands should run. It can be difficult to get this timing exactly right, especially if you're very picky about the execution time. In this post from Sameer Borate he shows you a PHP library that can help not only parse current cron configurations but also provides shortcuts for common timings (ex: "daily" or "weekly").

Working with cron scheduling can many times be a frustrating affair. Although setting a few cron jobs at one time can be easy, calculating cron dates in the future in code can get time consuming quickly. The PHP cron expression parser described here can parse a CRON expression, determine if it is due to run, calculate the next run date of the expression or calculate the previous run date of the expression. You can calculate dates far into the future or past by skipping n number of matching dates.

He includes some examples of putting the library to use to define a cron object based on an expression (either via a shortcut or an actual cron time expression). You can then check to see if the cron is "due" or perform some various operations about its run dates. This includes a formatted output of the previous run time, the next run time and the calculation of the next/previous run time based on a relative timestamp.

tagged: cron parser library example tutorial run due evaluation datetime

Link: http://www.codediesel.com/php/cron-expression-parser-in-php/

Anthony Ferrara:
Prefix Trees and Parsers
May 19, 2015 @ 10:13:18

Anthony Ferrara has a new post, following up from his previous look at tries and lexers, continuing along the path to apply what he learned to a HTTP routing system.

In my last post, Tries and Lexers, I talked about an experiment I was doing related to parsing of JavaScript code. By the end of the post I had shifted to wanting to build a HTTP router using the techniques that I learned. Let's continue where we left off...

He starts off with thinking that lexing and parsing the routes out into their respective tokens instead of breaking them up as many do (i.e. splitting on the slashes). He shows the results of this lexing and some parser code to handle these results and turn them into something useful. He did find that the current setup caused a lot of overhead (255 new states per character) so he optimizes the processing with a "default" trie but it was still pretty intensive.

He decided to go a different way at this point, opting for the radix tree structure instead. He includes the implementation of this tree for parsing the routes and his matching lexer updates. Finally he shows how to apply code generation to the results of these changes and how coming back to the "slash splitting" could help...

tagged: lexer parser example prefix tree radixtree route matching slashes

Link: http://blog.ircmaxell.com/2015/05/prefix-trees-and-parsers.html

Anthony Ferrara:
Tries and Lexers
May 18, 2015 @ 09:47:32

Anthony Ferrara has an interesting new post to his site talking about tries and lexers, two pieces of a puzzle that are used during script execution. In this case, he's tried his hand at writing a parser which, naturally, lead to needing a lexer.

Lately I have been playing around with a few experimental projects. The current one started when I tried to make a templating engine. Not just an ordinary one, but one that understood the context of a variable so it could encode/escape it properly. [...] So, while working on the templating engine, I needed to build a parser. Well, actually, I needed to build 4 parsers. [...] I decided to hand write this dual-mode parser. It went a lot easier than I expected. In a few hours, I had the prototype built which could fully parse Twig-style syntax (or a subset of it) including a more-or-less standards-compliant HTML parser. [...] But I ran into a problem. I didn't have a lexer...

He starts with a brief description of what a lexer is and provides a simple example of an expression and how it would be parsed into its tokens. He then talks about the trie, a method for "walking" the input and representing the results in a tree structure. He shows a simple implementation of it in PHP, iterating over a set of tokens and the array results it produces. He then takes this and expands it out a bit into a "lex" function that iterates over the string and compiles the found tokens.

From there he comes back to the subject of Javascript, pointing out that it's a lot looser than PHP in how it even just allows numbers to be defined. His testing showed a major issue though - memory consumption. He found that a regular expression method consumed too much and tried compiling out to classes instead (and found it much faster once the process was going).

tagged: lexer parser example javascript tries tree data structure

Link: http://blog.ircmaxell.com/2015/05/tries-and-lexers.html

Sherif Ramadan:
How to Write an Operator Precedence Parser in PHP
Jan 21, 2013 @ 11:21:22

Sherif Ramadan has a post looking at creating a better operator precedence parser in PHP. His example is a fully PHP implementation that takes equation strings and evaluates them to create the result.

Operator precedence parsers are very simple on the surface. So don’t feel in the least bit intimidated, because by the time you’ve read through this I hope to have you walk away with a solid foundation on how to write your very own operator precedence parser. The goal is to understand how to solve the problem of operator precedence parsing, and not necessarily to write your own parser. Learning how the problem can be solved is the most important thing to take away from this article.

He starts with an introduction to the concepts behind "operator precedence" including processing order and grouping. He also mentions infix and postfix (RPN) notations for handling different formats of equations. He used the "Shunting-yard Algorithm" and how it relates to handling the different parts of the equation, one at a time, in the correct order. He rest of the post is dedicated to the details of the execution in the tool, including code examples and the tokenization of the strings passed into it.

tagged: operator precedence parser string token shuntingyard algorithm

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Ibuildings Blog:
DPCRadio: Let's Build a Parser
Oct 24, 2012 @ 10:19:18

On the Ibuildings blog today they've released the latest episode of their DPC Radio podcast series as recorded at this year's Dutch PHP Conference (2012). In this new episode, they share Boy Baukema's session "Let's Build a Parser".

During this talk an introduction will be given to parsing. Terms like 'formal grammar', 'lexing / scanning', 'LL / LALR / PEG' will be explained and put into context. We will look at a recursive descent parsing as a practical way to parse languages. Finally the audience will be left with ways to get started with parsing structured text into memory.

You can listen to this latest episode either through the in-page player or by downloading the mp3. You can also see the slides from the presentation on Slideshare.

tagged: dpcradio podcast dpc12 parser boybaukema tutorial

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Sameer Borate's Blog:
Building a simple Parser and Lexer in PHP
Nov 17, 2011 @ 11:57:59

In a new post to his blog Sameer Borate shows how to create a lexer and parser in PHP to work directly with the tokens of a PHP script.

After looking around for a while [for a good resource on compilers] I settled for Terence Parr's Language Implementation Patterns. This is exactly what I needed – bit sized patterns on compiler and parser design with working code. The book provides a recipe style approach, gradually moving from simple to complex compiler/parser design issues. As I primarily work with PHP, I thought of porting some code to PHP to see how it works.

He shows examples using his custom tool to show a basic lexer output for a list and a complete listing of the code involved. Ultimately, though, he finds that PHP isn't overly suited to the task - anything more than his simple example could be more trouble than it's worth.

tagged: lexer parser tutorial language implement token

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Erik's Blog:
PHP implementation of an LALR1 parser generator
Jan 19, 2010 @ 13:03:57

Erik has posted about a new parser he's created, one based on the LALR parser method, that can be downloaded here [zip].

Parser tables, along with a lexer table, are the core constituents of any compiler's front end. This LALR parser generator is heavily based on my custom Php version of the map/reduce programming technique (functional programming). I will elaborate in future posts, on why I used a map/reduce basis to implement the parser generator.

Parsers organize a set of tokens based on the rules of a formal language the parser is given. Several of the grammar text files are included in the download so you can test it out for yourself. For more on the topic check out these two posts.

tagged: lalr1 parser generator grammar

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Erling Alf Ellingsen's Blog:
PHP Must Die
Jan 11, 2010 @ 13:49:41

In a (slightly inflammatory) post to his blog today Erling Alf Ellingsen shares why he thinks that "PHP must die", mostly due to some of the inconsistencies his has with other languages.

His examples include:

  • String vs. numeric handling
  • That PHP supports octal numbers "by accident"
  • A lexer bug with hex values
  • A parser bug involving the ternary operator

Comments on the post include those supporting the "die" opinion - that PHP just doesn't have it together like other languages - and those taking a bit more balanced approach on PHP's strengths and weaknesses.

tagged: opinion lexer parser octal ternary

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Phil Sturgeon's Blog:
Give CodeIgniter's Parser library more kick with Dwoo
Nov 24, 2009 @ 08:39:18

Phil Sturgeon has a new post to his blog talking about a way to give your CodeIgniter site a bit more "pep" - use his updated implementation of the Dwoo templating engine.

Not wanting to spend too much time re-inventing the wheel I had a look at existing templating engines and decided to use Dwoo. [...] So after a little head-scratching, we now have CodeIgniter-Dwoo. DwooParser? DwooIgniter? Whatever we call it, my new Dwoo implementation for CodeIgniter sits in with "the CodeIgniter way" perfectly and the only code you will need to change within your application is the Parser file and your view files. No changes to your controllers at all!

You can download the library from the code section of his site.

tagged: codeigniter parser library dwoo framework

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PHPImpact Blog:
PHP Simple HTML DOM Parser (jQuery Style)
Aug 08, 2008 @ 12:03:55

On the PHP::Impact blog today Federico points out a few HTML DOM parsers that work similar to jQuery:

Check out a previous tutorial of his for more information on using DOM parsers to scrape information from remote sites.

tagged: dom parser simple html jquery zendomquery phpquery

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