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5 years ago
In PHP 7.3+, and depending on the environment, there may be other constants that aren't ints which will cause errors with array_flip. I opted to go with a RegexException and a filter on gk's note.

<?php

class RegexException extends \Exception {

    public $errorCode;

    public function __construct(
        int $errorCode,
        string $additionalMessage = null
    ) {
        $this->errorCode = $errorCode;

        $errorMessage = $this->getErrorString($errorCode) ?? 'Unknown regex error';
        $additionalMessage = $additionalMessage ? " $additionalMessage" : '';

        parent::__construct("Regex error thrown: $errorMessage." . $additionalMessage);
    }

    /**
     * Gets an error string (const name) for the PCRE error code
     *
     * @param int $errorCode
     *
     * @return string|null
     */
    private function getErrorString(int $errorCode): ?string
    {
        $pcreConstants = get_defined_constants(true)['pcre'] ?? [];

        /*
         * NOTE: preg_last_error() returns an int, but there are constants
         * in PHP 7.3+ that are strings, bools, or otherwise. We can pretty
         * safely filter out the non-integers to fetch the appropriate
         * error constant name.
         */
        $pcreConstants = array_filter($pcreConstants, function ($code) {
            return is_int($code);
        });

        $errorStrings = array_flip($pcreConstants);

        return $errorStrings[$errorCode] ?? null;
    }

Usage:

throw new RegexException(preg_last_error());
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