Questions concerning /o regex modifier

Limbic~Region has asked for the wisdom of the Perl Monks concerning the following question:

All,
I am likely getting some of this wrong and I am sure some of the more knowledgeable monks will correct the inaccuracies. The question remains regardless.

The /o regex modifier is an old optimization before qr// existed. If you had a regex that needed variable interpolation before compiling but knew the variable would never change, you could use the /o modifier to only compile the regex once (/$regex/o). In fact, if you broke your promise and modified the variable, the regex still would not be recompiled leading to some potentially buggy code. See perlre for more information.

The qr// operator greatly improved things. See perlop for more details. In my opinion, this pretty much rended the need for /o moot. I am not the only one, diotalevi has /o is dead, long live qr//! to say on the matter as well as this.

Ok, I get it - backwards compatability and TIMTOWTDI. There is a lingering question. It is pointed out here that in newer versions, perl automatically detects when a variable is changed and only recompiles a regex when necessary. This is apparently not documented. I checked perldeltas from 5.6 to 5.9.4 though I may have missed it.

So why is this behavior not documented? Is there any reason not to deprecate /o? I am happy to perlbug a doc patch but I am certainly not the most qualified to do so.

Cheers - L~R

Comment on Questions concerning /o regex modifier

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Re: Questions concerning /o regex modifier by suaveant (Parson) on Nov 29, 2006 at 15:51 UTC
Well... there is that rare and special occasion when the variable changes and you don't want the regex to change... Remember we are talking about backwards compatibility and not good coding practices when I say this :) - Ant - Some of my best work - (1 2 3)	[reply]
Re^2: Questions concerning /o regex modifier by Limbic~Region (Chancellor) on Nov 29, 2006 at 15:55 UTC
suaveant, Yes for extremely old versions that did not support qr// it was useful. My question regarding deprecation stands. In very old versions the deprecated warning won't appear but in newer versions where it is not necessary it will. Makes good sense to me. Cheers - L~R	[reply]
Re^3: Questions concerning /o regex modifier by suaveant (Parson) on Nov 29, 2006 at 16:11 UTC
I could see deprecating it... especially since very little that gets deprecated ever actually goes away anyway. No old code would actually break but they would get warnings. I wonder how it is being handled in Perl6... maybe you just have to wait a bit. - Ant - Some of my best work - (1 2 3)	[reply]
Re^4: Questions concerning /o regex modifier by idsfa (Vicar) on Nov 29, 2006 at 17:09 UTC
Re: Questions concerning /o regex modifier by diotalevi (Canon) on Nov 29, 2006 at 18:00 UTC
If you interpolate without /o, perl will always do the work of stringifying everything you gave it and concatenating it all into a single string. If the current string matches the last string then the compiled regexp from last time is re-used. This is an implementation detail and would need justification to get documented. ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊	[reply]
Re^2: Questions concerning /o regex modifier by Limbic~Region (Chancellor) on Nov 29, 2006 at 18:29 UTC
diotalevi, Ok, but I am still don't understand when and why /o is useful. Would you be kind enough to fill in the blanks? Use /o when _____ Use qr// when _____ Rely on /$regex/ implementation details when _____ Do not use /o when _____ Do not use qr// when _____ Do not rely on /$regex/ implementation details when _____ Cheers - L~R	[reply]
Re^3: Questions concerning /o regex modifier by diotalevi (Canon) on Nov 29, 2006 at 21:30 UTC
Use /o when writing for a perl 5.005 and less. You are nano-optimizing to avoid the null operation of a pp_regcomp given a qr// object Use qr// when writing for a perl 5.6 and greater. You want the syntax affordance of being able to say `qr/\s/` rather than `"\\s"` (assuming your interpolating into some larger expression) Rely on /$regex/ implementation details when you've bothered to read the source and know exactly what it's doing Do not use /o when writing for humans to read (because almost no one understands what /o does) on perls 5.6 and greater ever. It's just bad style now. Do not use qr// when on perl 5.005 and lower you're writing a sub expression that's going to be interpolated elsewhere and you care that the regexp compiler takes longer to compile strings like '(?-xism:...)' slower than '...'. Do not rely on /$regex/ implementation details when you haven't read the source to pp_regcomp. ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊	[reply] [d/l] [select]
Re^4: Questions concerning /o regex modifier by Limbic~Region (Chancellor) on Nov 29, 2006 at 21:49 UTC
Re^2: Questions concerning /o regex modifier by tye (Sage) on Nov 29, 2006 at 18:36 UTC
/o is motivated by performance concerns. Significantly reducing the whole motivation for the existence of /o in the first place warrants documenting such a change. - tye	[reply]
Re^3: Questions concerning /o regex modifier by diotalevi (Canon) on Nov 29, 2006 at 21:19 UTC
That's qr// and it's documented. ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊	[reply]
Re^2: Questions concerning /o regex modifier by bunnyman (Hermit) on Nov 29, 2006 at 18:36 UTC
In other words, /o is still more efficient than leaving it out? Also, it is a sign (a flag, if you will) to humans that the variables in this expression will not (and should not) change.	[reply]
Re^3: Questions concerning /o regex modifier by diotalevi (Canon) on Nov 29, 2006 at 21:17 UTC
It's not a signal to 99% of the humans that will read it because almost no one gets what /o does. There's just the "Oh! Optimized! Ok, it's more better n' good!" It is faster to avoid doing the concatenation and string comparison work than not but those are cheap operations. If you were optimizing for that then you could just as well pass in a qr// object. ⠤⠤ ⠙⠊⠕⠞⠁⠇⠑⠧⠊	[reply]
Re^4: Questions concerning /o regex modifier by sgt (Deacon) on Dec 01, 2006 at 22:48 UTC