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Date: Fri, 25 Jul 2025 23:44:25 +0100
To: internals@lists.php.net
Subject: Re: [PHP-DEV] pcre extended character class support
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From: derick@php.net (Derick Rethans)

On 25 July 2025 23:17:43 BST, Niels Dossche <dossche=2Eniels@gmail=2Ecom> w=
rote:
>Hi internals
>
>On PHP 8=2E5-dev, we ship with pcre2lib 10=2E45=2E
>
>This includes a new opt-in feature called "PCRE2_ALT_EXTENDED_CLASS"=2E
>It enables the use of complex character set operations in accordance to U=
TS#18 (Unicode Technical Standard 18)=2E
>This means it becomes possible to nest character sets, perform set operat=
ions on them, etc=2E
>One example of such a set operation is a set subtraction, e=2Eg=2E the re=
gex "[\ep{L}--[QW]]" means "Unicode letters other than Q and W"=2E
>Or a more realistic example (inspired from [1]): the regex "[\p{Lu}--[0-9=
]]" matches all non-ASCII unicode numbers=2E
>You can also do ORs, ANDs, etc=2E
>
>The reason this is opt-in in pcre2lib, is because the interpretation of e=
xisting regexes may change=2E
>This standard is being adopted in other languages too, also opt-in, for e=
xample in JavaScript [1]=2E
>To expose this functionality in PHP, we also have to make it opt-in via a=
 modifier=2E
>
>In JavaScript, this is enabled via the /v modifier at the end of the rege=
x [1]=2E
>This does the same thing as the /u modifier, but extends it with this UTS=
#18 standard=2E
>We also already have /u in PHP that enables UTF-8 unicode mode=2E So we c=
ould do the same as JavaScript and add a /v modifier that extends /u and al=
so enables PCRE2_ALT_EXTENDED_CLASS=2E Technically, you don't need unicode =
processing for enabling PCRE2_ALT_EXTENDED_CLASS, but as it comes from a un=
icode standard (and that at least JavaScript does this too), it may make se=
nse to enable them both=2E
>
>The actual patch is trivial:
>```diff
>diff --git a/ext/pcre/php_pcre=2Ec b/ext/pcre/php_pcre=2Ec
>index 8e0fb2cce5f=2E=2E4a4727545ad 100644
>--- a/ext/pcre/php_pcre=2Ec
>+++ b/ext/pcre/php_pcre=2Ec
>@@ -718,6 +718,9 @@ PHPAPI pcre_cache_entry* pcre_get_compiled_regex_cach=
e_ex(zend_string *regex, bo
> 			case 'S':	/* Pass=2E */					break;
> 			case 'X':	/* Pass=2E */					break;
> 			case 'U':	coptions |=3D PCRE2_UNGREEDY;		break;
>+#ifdef PCRE2_ALT_EXTENDED_CLASS
>+			case 'v':	coptions |=3D PCRE2_ALT_EXTENDED_CLASS; ZEND_FALLTHROUGH;
>+#endif
> 			case 'u':	coptions |=3D PCRE2_UTF;
> 	/* In  PCRE,  by  default, \d, \D, \s, \S, \w, and \W recognize only AS=
CII
> 	   characters, even in UTF-8 mode=2E However, this can be changed by se=
tting
>
>```
>
>What do we think?
>
>[1] https://github=2Ecom/tc39/proposal-regexp-v-flag



Yes, please=2E

cheers
Derick