How to find out if a string is a PHP regex? [duplicate]

Question

Is there any way to do this, without using the error control operator on @preg_match ? :)

Because PHP requires that patterns are wrapped between a character, I was thinking to do it this way:

1. Get the first character.

2. Find the last occurence of this character in the string with:

   $rpos = strrpos($string, $string[0]);
   

3. Get the modifier list:

   $mods = substr($rpos, strlen($string));
   

4. Find out if there are any invalid modifiers in this list. If there are return false, otherwise regex appears to be valid.

How reliable is this solution? Are there any better ones?

Answer 1

I'm always using this function in the Symfony2 Finder component:

if (preg_match('/^(.{3,}?)([imsxuADU]*)$/', $expr, $m)) {
    $start = substr($m[1], 0, 1);
    $end   = substr($m[1], -1);

    if (($start === $end && !preg_match('/[*?[:alnum:] \\\\]/', $start)) || ($start === '{' && $end === '}')) {
        // It's a regex!
    }
}

Answer 2

In the commets it is already described in the short way. But you can check if the regex is okay by using the filter_var method

$string = "String to match";

if (filter_var($string, FILTER_VALIDATE_REGEXP, array("options" => array("regexp" =>  "/abc/")))) {
    echo "Regex is OK";
} else {
    echo "Regex not okay";
}