The /o in Ruby regex stands for "oh the humanity " | Hacker News Reader

The /o in Ruby regex stands for "oh the humanity "

jpcamara.com

186 pointstodsacerdoti42 comments

This is one of the features that Ruby cribbed directly from Perl. The Ruby documentation seems really bad, in particular “interpolation mode” is grievously misleading.

Perl’s documentation is far more clear about the consequences:

(https://perldoc.perl.org/perlop#Regexp-Quote-Like-Operators)

   o   Compile pattern only once.

  […]

  PATTERN may contain variables, which will be
  interpolated every time the pattern search is
  evaluated, except for when the delimiter is a
  single quote. […] Perl will not recompile the
  pattern unless an interpolated variable that
  it contains changes. You can force Perl to skip
  the test and never recompile by adding a /o
  (which stands for "once") after the trailing
  delimiter. Once upon a time, Perl would recompile
  regular expressions unnecessarily, and this
  modifier was useful to tell it not to do so,
  in the interests of speed. But now, the only
  reasons to use /o are one of:

  [reasons]

  The bottom line is that using /o is almost
  never a good idea.

In the days before Perl automatically memoized the compilation of regexes with interpolation, even back in the 1990s, it said,

  However, mentioning /o constitutes a promise
  that you won't change the variables in the
  pattern. If you change them, Perl won't even
  notice.

Perl 4’s documentation is briefer. It says,

(https://github.com/Perl/perl5/blob/perl-4.0.00/perl.man#L272...)

  PATTERN may contain references to scalar
  variables, which will be interpolated
  (and the pattern recompiled) every time the
  pattern search is evaluated. […] If you want
  such a pattern to be compiled only once, add
  an “o” after the trailing delimiter. This
  avoids expensive run-time recompilations, and
  is useful when the value you are interpolating
  won't change over the life of the script.

https://perldoc.perl.org/perlre

  o  - pretend to optimize your code, but actually introduce bugs

Just add this flag for odd versioned deployments, infinity job security

Nowadays, computers are super fast and super huge and super always available.

But even in the early 2000s, that was not the case. I know this b/c one of my first jobs out of college was to build a regex based system to analyze spam emails sent to big providers (like Yahoo and Hotmail (Microsoft)".

IIRC, we only had ONE Dell 2600 box to do all of the above with literally millions of emails and the same box did the parsing and the database storage (via MySql).

You REALLY learn what makes regexes efficient versus not and I remember reading about "/o" and testing it to see if it made a difference for what I was doing. I don't remember either way what the results were but this was def a blast from the past.

PS If you want to read more about the spam work back the, I have a Twitter thread about it here: https://x.com/alexpotato/status/1208948480867127296

This is a footgun. A language should strive not to add footguns. Every footgun you provide, somebody is going to blow their foot off with it, so that's a high price. If your language is popular it might be a lot of somebodies.

The opposite behaviour (we have a constant regular expression, we re-use it often but the tooling doesn't realise and so it's created each time we mention it) is not a footgun, it results in poor performance, and so you might want (especially in some managed languages) to just magically optimise this case, but if not you won't cause mysterious bugs. An expert, asked "Why is this slow?" can just fix it - you have to supply basic tools for that, but this flag is not a sensible tool.

Is it really though? There are tons of characters you can add to a regex that have difficult if not impossible to mentally comprehend impacts on the potential matches. That's why you need 100 test cases for every 10 characters you write in a regex. Regex itself could all be a footgun by this standard. No one in the history of no one has ever thought "why dont I just add a random character to my regex I don't need or understand" that's just boogie man level irrational fear if you think this has any bearing on the ease of use of ruby.

Ruby is a well-sharpened knife. Not everyone should be given a sharp knife though, especially children. And not all jobs need a sharp knife, like buttering toast. So I think it’s good for dull knives to exist as part of your tool belt. If we can only choose one language though, I’d rather it be a nimble, sharp one.

Regexes are not fundamentally hard. People make regexes hard by trying to parse things by sight rather than finding a spec. If you have a spec, and it can be parsed by a regular expression, they are pretty damn rote to implement.

If you aren’t working from a specified input grammar, the task is going to be borderline impossible no matter the tool and you’re going to have a bad time. If you aren’t working with a regular grammar, this is the wrong tool for the job and again you’re going to have a bad time.

A hint; if you find yourself using `.`, you are probably shooting yourself in the foot.

About 10 years ago I had to write a script to reconcile trade entries from various brokers back into our central system.

You are 100% correct on the "100 to 10" ratio on test cases.

PLUS, the ways in which broker files can break due to:

- random carriage returns

- different date formats

- time zones

- etc etc

and regexes become both great and terrifying at the same time.

One pattern I did find useful:

regex + if/then

e.g. if (regex is true) then if (regex2 is true) then

Sometimes you want to blow your foot off.

One of my favorites was Python’s datetime.time() object evaluating to True for every value except exact midnight, which is the sort of thing that makes fine sense when you think about the underlying implementation but is absolutely going to take a toe off of someone.

My favorite part about that one was it got to go through the full feature deprecation cycle before removal because several people argued in the bug thread about it that they were relying on that behavior in their systems.

In the 1990s, with the processing power of the time, /o was a reasonable compromise. The language later evolved to do the smart thing you describe, but you can't just remove features. A warning would be in order though.

It looks like an emoji for someone getting bashed in the head with a long stick. So that makes sense?

Unsurprisingly, `END {}` is also inherited from perl, tho I think it originally comes from awk.

Similarly unsurprisingly, with its BEGIN friend https://docs.ruby-lang.org/en/3.3/syntax/miscellaneous_rdoc....

In the spirit of "what's old is new again," PowerShell also has the same idea, and is done per Function with "begin", "process", "end", and "clean" stanzas that allow setup, teardown, for-each-item, and "finally" behavior: https://learn.microsoft.com/en-us/powershell/module/microsof...

Oh, that’s an interesting take. I’ve long been looking for newer developments on Awk’s clause structure, and this seems like an interesting take (though I’m unclear on whether I can have multiple begin/end clauses, which are the best thing about Awk’s version). It also finally connects this idea to something else in my mind—specifically advice[1] and CLOS’s :before/:after/:around methods[2]. (I guess Go’s defer also counts?)

[1] https://en.wikipedia.org/wiki/Advice_(programming)

[2] https://gigamonkeys.com/book/object-reorientation-generic-fu...

It seems not:

Given:

    function Fred {
        begin {
            echo "hello from begin1"
        }
        begin {
            echo "hello from begin2"
        }
        process {
            echo "does the magic"
        }
    }
    $bob = @("alpha" "beta")
    $bob | Fred

Then

    $ pwsh fred.ps1
    ParserError: /Users/mdaniel/fred.ps1:5
    Line |
       5 |      begin {
         |      ~~~~~~~
         | Script command clause 'begin' has already been defined.

As an old Perl programmer, I knew immediately what the /o would do. ;-)

I've always loved the recent[1] summary from `perlre`:

>o - pretend to optimize your code, but actually introduce bugs

1: I still think of it as a relatively new change, but it's from 2013: <https://github.com/Perl/perl5/commit/7cf040c1f649790a4040aec...>

It's older than that. The article links to this conversation about it in 2003: https://www.perlmonks.org/?node_id=256053

This is the same problem people have with closures, where it's unclear to the user whether the argument is captured by name or by value.

This isn't the same problem, because this is about whether the regex is instantiated each time the code around the regex is executed, or only the first time and cached for subsequent executions. The same could in theory happen with closures, but I haven't ever seen programming-language semantics where, for example, a function containing the definition of a closure that depends on an argument of that outer function, would use the argument value of the first invocation of the function for all subsequent invocations of the function.

For example, when you have

    fn f x = (y -> x + y)

then a sequence of invocations of f

    f 1 3
    f 2 6

will yield 4 and 8 respectively, but never will the second invocation yield 7 due to reusing the value of x from the first invocation. However, that is precisely what happens in the article's regex example, because the equivalent is for the closure value (y -> x + y) to be cached between invocations, so that the x retains the value of the first invocation of f — regardless of whether x is a reference by name or by value.

The parallel is apt, but regex /o is more like a closure that captures by value at declaration time rather than an ambiguity between capture strategies.

> Modifier o means that the first time a literal regexp with interpolations is encountered, the generated Regexp object is saved and used for all future evaluations of that literal regexp.

That is crystal clear to me. It means that on the next execution, the new values of the interpolation will be ignored; the regexp is now "baked" with the first ones.

Like this in C++:

  void fun(int arg)
  {
     static int once = arg;
  }

if we call this as f(42) the first time, once gets initialized to 42. If we then call it f(73), once stays 42.

There is a function in POSIX for once-only initializations: pthread_once. C++ compilers for multithreaded environments emit thread-safe code to do something similar to pthread_once to ensure that even if there are several concurrent first invocations of the function, the initialization happens once.

If you don’t like /o, you’re going to hate Perl’s /e

Love these sorts of deep dives, thanks!

It's kind of a cool feature. I like it.

Is it? I can't think of a non-contrived case where this would actually be useful.

And in any case where it would be useful, it seems like a better way to optimize would just be to refactor the regex out into a constant.

Actually, I have a way this would work well. If you’re interpolating a value that comes from configuration and wouldn’t change.

Example: /admin@#{Rails.config.x.domain}/io

But you’re right that a constant would be a lot more clear. “o” is a footgun.

The context is that this is a feature cribbed straight from perl, where where it's passed down from perl 4/pre-5.6, where compiled regexen weren't first-class values. Pretty much every use of it this century is a mistake.

An HTTP application server matching routes based on runtime configiuration (domains and whatnot) is not really that niche or contrived? Loads of other situations where input not changing during the thread/process lifecycle is part of a set of hot regexes large enough that explicitly compiling each is not a great experience.

I, for one, appreciate /o.

this is similar to the g modifier in javascript?

No, g is the global modifier so gives you multiple mateches rather than stopping on the first match encountered.

Seems par for the course for Ruby.

Im sorry but the classics never go out of style:

"Some people, when confronted with a problem, think 'I know, I'll use regular expressions.' Now they have two problems."

Yeah but it's kind of tired when it's being used every time someone makes a mistake with regex. I've used them extensively in my career and never once regretted it.

The problem with regexps is that “Sometimes a smart person, who has done the work, and knows how to leverage regular expressions correctly, decides they are appropriate for solving a problem where there is shared maintenance. Now, you have people who haven’t put in the work, and have been told repeatedly through ‘witty quips’ to not bother.”

The second problem being how to deal with all the extra time they just freed up?

> I didn’t recognize /o. It didn’t seem critically important to lookup yet.

> With nothing else to investigate, I finally looked up the docs for what the /o regex modifier does.

I'll probably never understand this mode of thinkning. But then again, Ruby programmers are, after all, people who chose to write Ruby.

> /o is referred to as “Interpolation mode”, which sounded pretty harmless.

Really? Those words sound quite alarming to me, due to personal reminiscences of eval.

Also, this whole "/o" feaure seems insane. If I have an interpolation in my regex, obviously I have to re-interpolate it every time a new value is submitted, or I'd hit this very bug. And if the value is expected to the same every time, then I can just compile it once and save the result myself, right? In which case, I probably could even do without interpolation in the first place.

“Compilation”, I think, is exactly right. This feature is less about interpolation than it is about compilation of a single regexp to be used many times. It’s just shrouded in confusing documentation that should say: “/o tells ruby to rewrite this code such that it refers to a new statically allocated regexp object.” And when you write it that way, you see how insane it is for a function call to be hoisted automatically like this, without an explicit, obvious, syntactic annotation.

The implications of "statically allocated" are less clear than if you'd just write "compiled only once".

It's a feature dating from the 1990s, when Perl (and I guess Ruby?) didn't have a way for the user to store a compiled regex, and this was a useful shortcut for a very specific optimization, which Ruby documented badly. Perl (and I guess Ruby?) later evolved in a way that made /o unnecessary, but the (now mis)feature remained.

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