PHP Regex Functions

Question

I am trying to validate form input data using PHP's preg_match function. I am a little confused of how to use it. If I want to validate say an alphanumeric string, I would use ^[0-9a-zA-Z ]+$ as the first parameter and the string we're validating as the second one. But how would I use preg_match to tell if it's valid or not? Would I do this:

if(preg_match("^[0-9a-zA-Z ]+$", $_POST['display_name'])){
  "String is valid";
} else {
  "String is not valid";
}

Or the other way around? I am currently using the if not preg_match if statement but it's returning false for some reason... I know this is probably an easy answer, but I cannot figure this out.

@Radu: I wasn't sure whether the space was a typo. The OP stated that they wanted "to validate say an alphanumeric string". — johnsyweb
– johnsyweb, Commented Oct 18, 2011 at 3:21
I think what you need to do first is to pick best answers on those 12 other questions without best answers. — netcoder
– netcoder, Commented Oct 18, 2011 at 3:28
it's returning false for some reason... You would have gotten a helpful warning, if you hadn't turned down error_reporting() — mario
– mario, Commented Oct 18, 2011 at 4:08

bumperbox · Accepted Answer · 2011-10-22 23:40:31Z

3

FALSE return from a preg_match indicates an error

you need to delimit your regex (see the leading and trailing / you can use other characters too

if (preg_match("/^[0-9a-zA-Z ]+$/", $_POST['display_name'])) {

edited Oct 22, 2011 at 23:40

answered Oct 18, 2011 at 3:13

bumperbox

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5 Comments

Ibu Over a year ago

escape? you mean add a delimiter

deceze Over a year ago

Just some nitpicking: a regex cannot return false, only a function like preg_match that uses a regex can. It also does't necessarily indicate an error, if preg_match returns false it means there was no match. preg_match does throw an error if the regex is invalid, but apparently the OP has turned off error reporting. :)

bumperbox Over a year ago

@deceze i have edited the returns false from regex to returns false from preg_match. according to the manual it says returns false on error, returns 0 for no match. although i have not run tests to verify that the manual is correct.

deceze Over a year ago

Oh, there you go. It slipped my mind that preg_match usually returns 0 or 1. So ignore that point of my picking. :)

ridgerunner Over a year ago

Not quite true... Yes, the PHP documentation for preg_match() says that it returns FALSE on error. Unfortunately, PHP versions 5.3.3 and below have a bug (#52732) where preg_match() does NOT return FALSE on error (it instead returns int(0), which is the same value returned in the case of a non-match). This bug was fixed in PHP version 5.3.4.

xdazz · Accepted Answer · 2011-10-18 03:15:12Z

1

You need add the delimiters of your pattern, like this:

preg_match("/^[0-9a-zA-Z ]+$/", $_POST['display_name'])

answered Oct 18, 2011 at 3:15

xdazz

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