Regular expressions

[Chris Angelico]

On Thu, Nov 5, 2015 at 11:13 AM, rurpy--- via Python-list
<python-list at python.org> wrote:
> On 11/04/2015 07:52 AM, Chris Angelico wrote:
>> On Thu, Nov 5, 2015 at 1:38 AM, rurpy wrote:
>>> I'm afraid you are making a category error but perhaps that's in
>>> part because I wasn't clear.  I was not talking about computer
>>> science.  I was talking about human beings learning about computers.
>>> Most people I know consider programming to be a higher level activity
>>> than "using" a computer: editing, sending email etc.  Many computer
>>> users (not programmers) learn to use regular expressions as part
>>> of using a computer without knowing anything about programming.
>>> It was on that basis I called them more fundamental -- something
>>> learned earlier which is expanded on and added to later.  But you
>>> have a bit of a point, perhaps "fundamental" was not the best choice
>>> of word to communicate that.
>>
>> The "fundamentals" of something are its most basic functions, not its
>> most basic uses. The most common use of a computer might be to browse
>> the web, but the fundamental functionality is arithmetic and logic.
>
> If one accepted that then one would have to reject the term "fundamental
> use" as meaningless.  A quick trip to google shows that's not true.

A quick trip to Google showed me that there are a number of uses of
the phrase, mostly in scientific papers and such. I've no idea how
that helps your argument.

> But string matching *is* a fundamental problem that arises frequently
> in many aspects of CS, programming and, as I mentioned, day-to-day
> computer use.  Saying its "only" for pattern matching is like saying
> floating point numbers are "only" for doing non-integer arithmetic,
> or unicode is "only" for representing text.  (Neither of those is a
> good analogy because both lack the important theoretical underpinnings
> that regular expressions have [*]).

String matching does happen a lot. How often do you actually need
pattern matching? Most of the time, you're doing equality checks - or
prefix/suffix checks, at best.

> There would be far fewer computer languages, and they would be much
> more primitive if regular expressions (and the fundamental concepts
> that they express) did not exist.

So? There would also be far fewer computer languages if braces didn't
exist, because we wouldn't have the interminable arguments about
whether they're good or not.

> To be sure, I did gloss over Michael Torries' point that there are
> other concepts that are more basic in the context of learning
> programming, he was correct about that.
>
> But that does not negate the fact that regexes are important and
> fundamental.  They are both very useful in a practical sense (they
> are even available in Microsoft Excel) and important in a theoretical
> sense.  You are not well rounded as a programmer if you decline to
> learn about regular expressions because "they are too cryptic", or
> "I can do in code anything they do".

You've proven that they are important, but in no way have you proven
them fundamental. A regular expression library is the ideal solution
to the problem "I want to let my users search for patterns of their
own choosing". That's great, but it's only one specific class of
problem.

> I think the constant negative reception the posters receive here when
> they ask about regexes does them a great disservice.
>
> By all means point out that python offers a number of functions that
> can avoid the need for using regexes in simple cases.  Even point out
> that you (the plural you) don't like them and prefer other solutions
> (like writing code that does the same thing in a more half-assed bug
> ridden way, the posts in this thread being a case in point.)
>
> But I really wish every mention of regexes here wasn't reflexively
> greeted with a barrage of negative comments and that lame "two problems"
> quote, especially without an answer to the poster's regex question.

When has that happened? Usually there'll be at least two answers - one
that uses a regex and one that doesn't - and people get to read both.

>> Sure, you can
>> abuse that into a primality check and other forms of crazy arithmetic,
>> but it's not what they truly do. I also would not teach regexes to
>> people as part of an "introduction to computing" course, any more than
>> I would teach the use of Microsoft Excel, which some such courses have
>> been known to do. (And no, it's not because of the Microsoftness. I
>> wouldn't teach LibreOffice Calc either.) You don't need to know how to
>> work a spreadsheet as part of the basics of computer usage, and you
>> definitely don't need an advanced form of text search.
>
> Seems to me that clearly depends on the intent of the class, the students
> goal's, what they'll be studying after the class, what their current
> level of knowledge is, etc.  Your scenario seems way too under-specified
> to say anything definitive.  And further, the pedagogy of CS (or of any
> subject of education) is not "settled science" and that kind of question
> almost never has a clear right/wrong answer.

Uhh, "introduction to computing". What's the current level of
knowledge? Close to zero. That's the whole point of an introductory
class. It's a place where you teach the basics.

> This list is not a class.  If someone comes here with a question about
> Python's regexes they deserve an answer and not be bombarded with reasons
> why they shouldn't be using regexes beyond mentioning some of the alternatives
> in a "oh, by the way" way.  (And yes, I recognize in this case the OP did
> get a good answer from MRAB early on.)

"I want to swim from Sydney to Los Angeles, but my gloves keep wearing
out half way across the Pacific. How can I make my gloves strong
enough to get me to LA?"

Response 1: "If you use industrial-strength gloves and go via Papua
New Guinea, you can double up the gloves and swim to LA."

Response 2: "Swimming across the Pacific is a bad idea. Have you
considered taking a boat or plane instead?"

Which is the more helpful response? You can go ahead and assume the OP
always knows best; I'm going to at least offer some alternatives.

ChrisA