How to check if a given string is valid Regex?

Question

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Test if a regular expression is a valid one in PHP

 <?php 

    $subject = "PHP is the web scripting language of choice.";    
    $pattern = 'sssss';

    if(preg_match($pattern,$subject))
    {
        echo 'true';
    }
    else
    {
        echo 'false';
    }

?>

The above code gives me warning because the string $pattern is not a valid regular expression.

If I pass valid regular expression then it works fine.....

How can i check $pattern is valid regular expression?

See here: http://stackoverflow.com/questions/7095238/an-invalid-regex-pattern — Bart Friederichs, Jan 10 '13 at 07:43
or, better, here: http://stackoverflow.com/questions/362793/regexp-that-matches-valid-regexps — k102, Jan 10 '13 at 07:44
or this one: http://stackoverflow.com/questions/8825025/test-if-a-regular-expression-is-a-valid-one-in-php — Hvordan har du det, Jan 10 '13 at 07:45
this also helps:: http://stackoverflow.com/questions/10778318/test-if-a-string-is-regex — Sudhir Bastakoti, Jan 10 '13 at 07:46
Possibly least useful question + accepted answer as first result in google. But at least plenty of useful links... — Andrew, Jan 21 '16 at 02:48

score 5 · Answer 1 · answered Jan 10 '13 at 10:28

You could write a function that throws an error if there is something wrong with the Regexp. (Like it should have been in my opinion.) It is bad practice to use the @ to suppress warnings, but if you replace it with a thrown exception it should be ok.

function my_preg_match($pattern,$subject)
{
    $match = @preg_match($pattern,$subject);

    if($match === false)
    {
        $error = error_get_last();
        throw new Exception($error['message']);
    }
    return false;
}

then you can check if the regexp is correct with

$subject = "PHP is the web scripting language of choice.";    
$pattern = 'sssss';

try
{
    my_preg_match($pattern,$subject);
    $regexp_is_correct = true;
}
catch(Exception $e)
{
    $regexp_is_correct = false;
}

Bart Friederichs · Answer 2 · 2013-01-10T08:57:31.340

0

Use the === operator:

<?php 

    $subject = "PHP is the web scripting language of choice.";    
    $pattern = 'sssss';

    $r = preg_match($pattern,$subject);
    if($r === false)
    {
        // preg matching failed (most likely because of incorrect regex)
    }
    else
    {
        // preg match succeeeded, use $r for result (which can be 0 for no match)
        if ($r == 0) {
            // no match
        } else {
            // $subject matches $pattern
        }
    }

?>

edited Jan 10 '13 at 08:57

answered Jan 10 '13 at 08:07

Bart Friederichs

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I fixed the syntax error (case of copy-paste mistake). And my answer gives a way of detecting if the regex is incorrect (which is was the OP asked for). – Bart Friederichs Jan 10 '13 at 08:35
@loler how exactly? `preg_match($pattern, $subject);` is perfectly valid PHP. – Bart Friederichs Jan 10 '13 at 08:37
@loler apparently you are misreading the OP's question. He **says** the regex is invalid. This is not a syntax error, it's a warning. – Bart Friederichs Jan 10 '13 at 08:43
@loler no. OP's code also drops in the "false" part if there is no match. Which is something else than a regex error. Check this: http://nl3.php.net/manual/en/language.operators.comparison.php and the remark on preg_match's documentation page: "This function may return Boolean FALSE, but may also return a non-Boolean value which evaluates to FALSE." – Bart Friederichs Jan 10 '13 at 08:55

score -1 · Accepted Answer · edited May 23 '17 at 12:00

-1

You can just wrap preg_match with try catch, and consider result false, if it throws exception.

Anyway, you can take a look at regular expression to detect a valid regular expression.

edited May 23 '17 at 12:00

Community

1
1

answered Jan 10 '13 at 08:24

loler

2,594
1
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2

Adding `@` before functions to suppress warnings / errors is Bad Practice. It will hide possible bugs from your code. – Bart Friederichs Jan 10 '13 at 08:36

How to check if a given string is valid Regex?

3 Answers3