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

Answer 1

PHP has progressed quite a bit since this question was first asked (and answered). You can now (PHP 5.2+) 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){
        //Silence the error by using a @

        return str_replace("preg_match(): ", "", error_get_last()["message"]);
        //Make it prettier by removing the function name prefix
    }
    return NULL;
}

Demo