preg_last_error

PHP 8 has a native function to retrieve the actual error message, so these helper functions are no longer necessary. https://www.php.net/preg_last_error_msg <?php preg_match('/(?:\D+|<\d+>)*[!?]/', 'foobar foobar foobar'); if (preg_last_error() !== PREG_NO_ERROR) { echo preg_last_error_msg(); // Prints "Backtrack limit exhausted" } ?>

In PHP 5.5 and above, getting the error message is as simple as: <?php echo array_flip(get_defined_constants(true)['pcre'])[preg_last_error()];

Here is a more advanced function to convert an error code to text: <?php function preg_errtxt($errcode) { static $errtext; if (!isset($errtxt)) { $errtext = array(); $constants = get_defined_constants(true); foreach ($constants['pcre'] as $c => $n) if (preg_match('/_ERROR$/', $c)) $errtext[$n] = $c; } return array_key_exists($errcode, $errtext)? $errtext[$errcode] : NULL; } ?>

Here is a correction to one of the previous "get error message" snippets. It filters out the non-error codes which stops "Warning: array_flip(): Can only flip STRING and INTEGER values!" from being emitted due to ["PCRE_JIT_SUPPORT"]=>bool(true): <?php echo array_flip(array_filter(get_defined_constants(true)['pcre'], function ($value) { return substr($value, -6) === '_ERROR'; }, ARRAY_FILTER_USE_KEY))[preg_last_error()];

If you use T-Regx library, to get the last constant, you can use <?php echo preg_last_error_constant(); // 'PREG_BAD_UTF8_ERROR'

The above function pcre_error_deocde [sic] is not correct - not all of the used constants are errors constants. For example, when the error is actually PREG_BAD_UTF8_ERROR, the function outputs the text for PREG_SPLIT_OFFSET_CAPTURE.

Take it into account before use this function. My php --version PHP 5.6.29 (cli) (built: Dec 8 2016 09:19:46) Copyright (c) 1997-2016 The PHP Group Zend Engine v2.6.0, Copyright (c) 1998-2016 Zend Technologies On Linux Fedora 23 4.8.13-100.fc23.x86_64 #1 SMP Fri Dec 9 14:51:40 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux <?php // @see http://stackoverflow.com/questions/4440626/how-can-i-validate-regex // returns true because there is no opening "(" for the closing ")" var_dump(preg_match('~InvalidRegular)Expression~', null) === false); // this SHOULD be something different from 0, but... var_dump(preg_last_error()); ?>

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());

Just an addition to my previous note: In unicode mode (with "u" modifier), PREG_BAD_UTF8_ERROR only reflects errors in the subject string. If the pattern itself contains invalid characters, preg_match() (or preg_match_all()) returns false but preg_last_error() returns 0 indicating PREG_NO_ERROR. Instead, php issues a warning: "preg_match(): Compilation failed: invalid UTF-8 string at offset 0"

Getting error as text (small update): <?php echo array_flip(array_filter(get_defined_constants(true)['pcre'], function($v) { return is_integer($v); }))[preg_last_error()];