Why does my RegExp return false with proper number of characters?

$gi = filter_var(filter_var($_POST['group_id'], FILTER_SANITIZE_STRING),
        FILTER_VALIDATE_REGEXP, array(
           "options"=>array("regexp"=>"/^[a-zA-Z0-9_](?=[^\s]+$).{5,20}$/")));

That is the code I'm using. This is a PHP & MySQL application. The problem is that it is returning FALSE with anything 5 characters or shorter. If I change the '5' to a '6', then anything 6 characters or shorter returns FALSE. Why is this?

Also, the following code behaves the same:

$gi = filter_var($_POST['group_id'], FILTER_VALIDATE_REGEXP,
        array("options"=>array(
           "regexp"=>"/^[a-zA-Z0-9_](?=[^\s]+$).{5,20}$/")));

I'm under the impression that my regexp is 1) limiting entries to letters, numbers, and underscores, 2) prohi开发者_运维知识库biting whitespace, and 3) requiring atleast 5 characters and setting a maximum of 20 characters.

Your regex:

/^[a-zA-Z0-9_](?=[^\s]+$).{5,20}

is matching 5 characters, but only after it matches 1 character within [a-zA-Z0-9_], and then on to the 5 characters. So at minimum you need a 6 character string to match.

Based on your last paragraph, something like the following is more to what you're looking for:

/^[0-9a-zA-Z_]{5,20}$/

EDIT: Corrected thanks to @Allan, not quite in to the cognitive thinking pattern yet, coffee deprivation. ;-)

/.{5,20}/

This piece of RegEx is responsible as you are asking the engine to match AT LEAST 5 characters (Any characters) and 20 characters if max. Fix that piece accordingly.

Hope it helps :)

EDIT: Sorry, didnt read the last part of the question.

1) Wrong. The ^ operator negates whatever is in front, so the first char CAN NOT be any letter, any number nor underscore [Replace that with /^[\w_]/ if you really want that overkill filtering] 2) Right. There cannot be whitespaces before that end of line. 3) Pretty much whats above the EDIT ;)

For more regex info go to http://regular-expressions.info