Re: [DISCUSSION] Validating regex pattern

> On Oct 1, 2025, at 4:01 AM, Alexandre Daubois <alex.daubois+php@gmail.com> wrote: > > Hi everyone, > > I stumbled across the following issue, proposing to add a way to validate regex. [1] > > There is currently no way of knowing if a regex pattern is valid, apart from writing clunky > code. [2] Could you expand on how you define clunky here? I am curious, because I validate PCRE patterns like this using the existing mechanisms. function is_valid_preg_pattern( $pattern ) { $is_valid = true; set_error_handler( function () use ( &$is_valid ) { $is_valid = false; return true; }, E_WARNING ); preg_match( $pattern, '' ); restore_error_handler(); return $is_valid; } And this is based on the note in the man page for preg_match()… If the regex pattern passed does not compile to a valid regex, an E_WARNING <dfile:///Users/dmsnell/Library/Application%20Support/Dash/DocSets/PHP/PHP..docset/Contents/Resources/Documents/www.php.net/manual/en/errorfunc.constants.html#constant.e-warning> is emitted. Given the possible error conditions, I thought this was comprehensive and the only way for the E_WARNING to trigger when provided with the empty string is if the pattern fails to compile. Given the concerns that Juliette raised with regards to multiple regex engines, this one seems like it should be universal too. If there’s something clunky about this, it would aid my curiosity to learn. The $errstr parameter can also be inspected to ensure that it starts with preg_match() if there’s a chance any other warning could conflate with an unrecognized pattern. Caveat: I’m not aware of any performance implications for calling set_error_handler() like this. > > Two propositions emerged from the issue: either create a dedicated "preg_validate()" > function, or add a new flag to "filter_var()", namely FILTER_VALIDATE_REGEX_PATTERN. > > I would be in favor of the latter. The approach and implementation would surely be simpler. I > don't feel like we should do advanced error management. Knowing if a pattern is valid or not > would suffice for the vast majority of cases. > > I don't think the second approach would require an RFC. Christoph thinks that this should > at least be announced on the mailing list, so here we are. > > Looking forward to your feedback. > > — Alexandre Daubois > > [1] https://github.com/php/php-src/issues/9289 > [2] > https://stackoverflow.com/questions/4440626/how-can-i-validate-regex Warmly, Dennis Snell