How do I catch an invalid preg_match pattern?

Question

I am writing a PHP script that accepts a regular expression pattern from the user which is used by preg_match(). How can I check that the pattern is valid?

Answer 1

According to the docs,

preg_match() returns FALSE if an error occurred.

the problem is that it will also throw a warning.

One way to deal with this is is to suppress the output of the error message, catch the return value, and output the error using error_get_last() if it was false.

Something like

$old_error = error_reporting(0); // Turn off error reporting

$match = preg_match(......);

if ($match === false) 
 {
   $error = error_get_last();
   echo $error["message"];
 }

error_reporting($old_error);  // Set error reporting to old level

You may not need the error reporting bit in a production environment - it depends on your setup.

Answer 2

Just test it. preg_match() will return FALSE if the pattern is not valid.

Return values: 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_all() on the contrary will continue until it reaches the end of subject. preg_match() returns FALSE if an error occurred.

Answer 3

You can use preg_last_error() to get responses. Detail page:

http://php.net/manual/en/function.preg-last-error.php

Thanks!

Answer 4

One simple solution is to suppress the warning with an "@" and then check the error afterwards:

@preg_match($match, $ip);
if ( preg_last_error() != PREG_NO_ERROR ) {
   echo("<p>Syntax error in regular expression ".htmlentities($match)."</p>\n");
}

The string you are searching for ($ip in this case) and the preg_match return value - does not matter if all you want to do is check the syntax of the regular expression.

Answer 5

I thought I had seen a method for this in MRE; turns out it was one Friedl wrote himself. Here's the listing.

Answer 6

PHP has progressed quite a bit since this question was first asked (and answered). Since PHP 5.2 you can simply write the following to, not only test if the regular expression is valid, but to get the detailed error message if it's not:

if(@preg_match($pattern, '') === false){
   echo error_get_last()["message"];
}

Placed in a function

/**
 * Return an error message if the given pattern argument or its underlying regular expression
 * are not syntactically valid. Otherwise (if they are valid), NULL is returned.
 *
 * @param $pattern
 *
 * @return string|null
 */
function regexHasErrors($pattern): ?string
{
    if(@preg_match($pattern, '') === false){
        return str_replace("preg_match(): ", "", error_get_last()["message"]);
        //Make it prettier by removing the function name prefix
    }
    return NULL;
}

Demo

Answer 7

if (preg_match($regex, $variable)) {
    echo 'Valid';
}
else {
    echo 'InValid';
}