Perl One-Liners Explained, Part VII: Handy Regular Expressions

http://www.catonmat.net/blog/perl-one-liners-explained-part-seven/

by pkrumins • 15 years ago

64 14 15 years ago

tikhonj• 15 years ago

I think this collection clearly shows the simplicity and utility of short, potentially imperfect regular expressions. Reading and writing expressions like this--even if you never use them in your code--is a skill that almost every programmer would benefit from.

zby2• 15 years ago

These regexps might be useful as first approximations - but they are not as robust as the article suggest they are (for example numbers can contain decimal point or decimal comma depending on your language, parsing email addresses is a subject for an essay and I've heard the correct Perl regexp for that takes about a page of text). This article is harmful - because those who can benefit from this kind of basic regexp examples are also those who will not understand the limitations.

jsrn2• 15 years ago

> because those who can benefit from this kind of basic regexp examples are also those who will not understand the limitations.

I share your sentiment. The article is certainly useful for learning regexps. But IMHO it should also point to the correct way of doing things - often, the correct way is using a module and thus the resulting code is not much longer than the code in the article. For email address validation:

    use Email::Valid;
    print (Email::Valid->address('john@example.com') ? 'valid' : 'invalid');

as a oneliner:

   perl -MEmail::Valid -E"say (Email::Valid->address('john@example.com') ? 'valid' : 'invalid');"

Other than installing the Email::Valid module with a

    cpanm Email::Valid

it is not much longer than the example in the article.

https://metacpan.org/module/Email::Valid

DanielShir• 15 years ago

I guess you guys just read the code and not the text:

`Notice that I say "looks like". It doesn't guarantee it is an email address.`

And yeah, you can find the full regular expression in the back of one of O'Reilly's Perl books (the regular expression handbook I believe).

It's nice to see perl code from time to time, even if it's just one line :)

pkrumins2• 15 years ago

Good point. I updated article with a note about Email::Valid.

JoachimSchipper• 15 years ago

From http://www.ex-parrot.com/pdw/Mail-RFC822-Address.html:

Mail::RFC822::Address is a Perl module to validate email addresses according to the RFC 822 grammar. (...) Implementing validation with regular expressions somewhat pushes the limits of what it is sensible to do with regular expressions, although Perl copes well:

    (?:(?:\r\n)?[ \t])*(?:(?:(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(?:\r\n)?[ \t]
    )+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[ \t]))*"(?:(?:
    \r\n)?[ \t])*)(?:\.(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\031]+(?:(?:(
    ?:\r\n)?[ \t])+|\Z|(?=[\["()<>@,;:\\".\[\]]))|"(?:[^\"\r\\]|\\.|(?:(?:\r\n)?[
    \t]))*"(?:(?:\r\n)?[ \t])*))*@(?:(?:\r\n)?[ \t])*(?:[^()<>@,;:\\".\[\] \000-\0

    ** SNIP: 76 lines **

    ?:\r\n)?[ \t])*))*)?;\s*)

vph• 15 years ago

This is really the essence of Perl and also its "problem".

If you need a book to explain one-liners, it will be challenge to comprehend a program with 20 such one-liners.

JadeNB2• 15 years ago

I approach these articles as a way to show off the expressive abilities of a language, not to provide code snippets for cut-and-paste.

As others have mentioned (and as the articles themselves repeatedly mention), most of the time, if you have a common problem, you should find and use a CPAN module already built to solve it.

I have never used a code snippet from a one-liner compilation in one of my programs, but I have learned a lot of new constructs by reading them.

EDIT: Or, what tikhonj (http://news.ycombinator.com/item?id=3219843) said.

fennecfoxen• 15 years ago

Man, I like perl, and learning how to operate these sorts of regular expressions is useful, but a lot of the the "one-liners" are infested with magic variables and the sort of freakish syntactical constructs which give the language a bad name.

Good Perl is basically 72% of Ruby. (Less syntactic sugar. Less-structured reflection. And mildly crufty sigils - not that you'll notice those after your second week, though.)

JadeNB2• 15 years ago

In #117, the sentence ends too:

    The second regex matches <em>hello</em> because.

pkrumins2• 15 years ago

Fixed now!

Perl One-Liners Explained, Part VII: Handy Regular Expressions

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