Test if a regular expression is a valid one in PHP

Question

I am writing a form validation class and wish to include regular expressions in the validation. Therefore, the regex provided isn't guaranteed to be valid.

How can I (efficiently) check that the regex is valid?

Answer 1

Use the pattern in your preg_* calls. If the function returns false there is likely a problem with your pattern. As far as I know this is the easiest way to check if a regex pattern is valid in PHP.

---

Here's an example specifying the right kind of boolean check:

$invalidPattern = 'i am not valid regex';
$subject = 'This is some text I am searching in';
if (@preg_match($invalidPattern, $subject) === false) {
    // the regex failed and is likely invalid
}

Answer 2

When you have error reporting on, you can't get away with simply testing the boolean result. If the regex fails warnings are thrown (i.e. 'Warning: No ending delimiter xxx found'.)

What I find odd, is that the PHP documentation tells nothing about these thrown warnings.

Below is my solution for this problem, using try, catch.

//Enable all errors to be reported. E_WARNING is what we must catch, but I like to have all errors reported, always.
error_reporting(E_ALL);
ini_set('display_errors', 1);

//My error handler for handling exceptions.
set_error_handler(function($severity, $message, $file, $line)
{
    if(!(error_reporting() & $severity))
    {
        return;
    }
    throw new ErrorException($message, $severity, $severity, $file, $line);
});

//Very long function name for example purpose.
function checkRegexOkWithoutNoticesOrExceptions($test)
{
    try
    {
        preg_match($test, '');
        return true;
    }
    catch(Exception $e)
    {
        return false;
    }
}