Advanced Regular Expressions

http://www.smashingmagazine.com/2009/05/06/introduction-to-advanced-regular-expressions/

by pchristensen • 17 years ago

47 25 17 years ago

smashingmagazine.com

Hexstream• 17 years ago

The first four of these I wouldn't call "advanced"...

mncaudill• 17 years ago

Welcome to Smashing Magzine.

masomenos• 17 years ago

not for full-time programmers, but SM's audience seems to be more designers who may dip into code now and again

lucumo2• 17 years ago

And yet I was pleasantly surprised. I'm quite used to finding "advanced" articles on various programmic topics and finding that everything said in them is so basic that I can't learn even the smallest piece of information from it.

This one still had some quite interesting pieces of information that I hadn't heard of or hadn't studied in detail yet.

emmett• 17 years ago

How did this get to the top of HN? It seems like a boring blogospam rehash of a bunch of very basic CS concepts, not even well written...

stcredzero• 17 years ago

...basic CS concepts...

Something which is ironic, and also gets back to basic CS concepts, is that modern scripting languages have added so much power to "Regular Expressions," that they are no longer strictly Regular Expressions. They are no longer equivalent to Finite Automata. "Regular Expressions" in many cases are outright Turing Complete.

And so the abuse of our field's terminology continues.

DEinspanjer5• 17 years ago

DFA vs NFA. I thought it was humorous that the most important features of modern regex engines are the things that prevent them from meeting the criteria of the original definition for a regex engine. And of course, with a true DFA regex engine, you'll never suffer from catastrophic backtracking or exponential performance degradation with a bad regex or bad subject string.

10ren2• 17 years ago

I wish HN had downvotes for articles instead of harsh top comments.

mcav• 17 years ago

Or if the "flag" button made posts' gravity higher so that they fell to the bottom sooner.

Poleris• 17 years ago

I find when I move across languages (which happens frequently) or even tools (like grep), regexp implementation often differs enough to throw me off and introduce subtle bugs.

Is there a page that shows a mapping between different implementations of regular expressions and which languages and applications they're used in? Is there a better way to figure these things out than hunt for documentation every time?

gustavo_duarte3• 17 years ago

I've written an automated tool to convert Vim-style regexes to Perl-style. This is kind of a dramatic case as the dialect is very different, but it was a fun thing to write.

staunch• 17 years ago

Can you make vim use pcre?

DEinspanjer5• 17 years ago

Is it bi-directional? Is it on the vim.org website somewhere? I find Vim's escape heavy syntax cumbersome and I frequently wish I could write a quick Perl syntax regex and convert it to Vim.

Of course, Vim is one of the few regex engines to support variable width negative look-behinds, so I guess that counts for something. :)

zouhair• 17 years ago

sudo aptitude install txt2regex

0xdefec8• 17 years ago

indeed. grep -P is your friend. (except older grep versions seem to segfault on me constantly with various -P regexes)

DEinspanjer5• 17 years ago

A tool I find useful enough to warrant purchasing and running in a VM or Wine is RegexBuddy. It is great for debugging / optimizing regexes and supports several different language flavors. Still waiting for Vim and PHP flavors though. :) It allows you to generate code snippits in several languages that use the regex you constructed, and will assist you in performing cross-language translations.

viggity• 17 years ago

I'm a big fan of letting named groups being my documentation, instead of using the # comment notation.

DEinspanjer5• 17 years ago

Named groups have their use and are nice when the language supports them. There is a lot to be said for the (?xi) flags to allow a complicated regex to be indented and commented usefully.

Gompers• 17 years ago

Consider the word ‘smashing’. Using the above regular expression, the regex engine will first try to match the pattern ‘hi’ in ‘smashing’. It will not find a match.

... what?

10ren2• 17 years ago

I hadn't seen recursive regex before (almost a tautology). The operator in the article, "(R?)", isn't in Perl (v5.8.8).

DEinspanjer5• 17 years ago

Yeah. PHP syntax. From what I understand, they stuffed it in there mostly to allow easier processing of nested HTML tags and such.

lucumo2• 17 years ago

I did some experimenting with recursive regexes after reading this article. I hadn't heard of it before and it sounded like an interesting concept.

However, captures in the recursion can't be retrieved after it gets returned. This may not be surprising, but it does reduce the power of recursion somewhat.

tjpick• 17 years ago

coz you can't expect anyone using PHP to, you know, write a proper parser...

Advanced Regular Expressions

Related Stories