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To: Rasmus Schultz <rasmus@mindplay.dk>,
 PHP internals <internals@lists.php.net>
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Date: Wed, 1 Mar 2017 16:42:38 +0100
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Subject: Re: [PHP-DEV] PCRE caching
From: ivan.enderlin@hoa-project.net (Ivan Enderlin)

Hello,

We can also use a LRU caching strategy with a pre-defined (or 
user-defined) number of expressions to keep in the cache. This would 
also be a good idea to track number of times a regular expression is 
used. If this number reaches a certain threshold, then we could 
automatically re-compile it with the `S` modifier.

I agree there is a missed optimisation here. I share your feeling.

Regards.


On 01.03.17 16:35, Rasmus Schultz wrote:
> Hey internals,
>
> I was wondering whether or how PCRE regular expression get parsed and
> cached, and I found this answer on Stack Overflow:
>
> http://stackoverflow.com/questions/209906/compile-regex-in-php
>
> Do I understand this correctly, that:
>
> 1. All regular expressions are hashed and the compiled expression is cached
> internally between calls.
>
> 2. The /S modifier applies more optimizations during compile, but caching
> works the same way.
>
> 3. Compiled expressions are not cached between requests.
>
> If so, this seems far from optimal.
>
> Every unique regular expression needs to be compiled during every request,
> right?
>
> So with FPM, or with long-running apps, we're missing an opportunity to
> optimize by caching between requests.
>
> And with long-running apps, we're caching every dynamic regular expression,
> which could harm (memory overhead) more than help.
>
> Ideally, shouldn't we have (like some engines/languages) a switch to enable
> caching?
>
> The run-time can't know if a given regular expression is dynamic or static,
> can it? It's just a string either way - so without a switch, you're either
> committing compiled dynamic expressions to the cache unnecessarily, and/or
> missing an opportunity to cache between requests in long-running apps or
> under FPM.
>
> I think most apps use quite a lot of regular expression for validation etc.
> so maybe there's a missed optimization opportunity here?
>
> Cheers,
>    Rasmus Schultz
>