How to validate a regex with PHP

Question

I want to be able to validate a user's inputted regex, to check if it's valid or not. First thing I found with PHP's filter_var with the FILTER_VALIDATE_REGEXP constant but that doesn't do what I want since it must pass a regex to the options but I'm not regex'ing against anything so basically it's just checking the regex validity.

But you get the idea, how do I validate a user's inputted regex (that matches against nothing).

Example of validating, in simple words:

$user_inputted_regex = $_POST['regex']; // e.g. /([a-z]+)\..*([0-9]{2})/i

if(is_valid_regex($user_inputted_regex))
{
    // The regex was valid
}
else
{
    // The regex was invalid
}

Examples of validation:

/[[0-9]/i              // invalid
//(.*)/                // invalid
/(.*)-(.*)-(.*)/       // valid
/([a-z]+)-([0-9_]+)/i  // valid

Answer 1

Here's an idea (demo):

function is_valid_regex($pattern)
{
    return is_int(@preg_match($pattern, ''));
}

preg_match() returns the number of times pattern matches. That will be either 0 times (no match) or 1 time because preg_match() will stop searching after the first match.

preg_match() returns FALSE if an error occurred.

And to get the reason why the pattern isn't valid, use preg_last_error.