Function preg_match() returns nothing

Question

I want to use preg_match() in my code, but the result is nothing ... (or null or empty ?)

$domain = "stackoverflow.com";

$uriToTest = "http://stackoverflow.com/";

$pattern = "/^http(s)?://(([a-z]+)\.)*".$domain."/";

echo preg_match($pattern, $uriToTest);

What is the problem?

Answer 1

If you take a look at your pattern, it's this

/^http(s)?://(([a-z]+)\.)*stackoverflow.com/

The delimiter is used as a matching character, and if you had errors turned on, you'd get a "Unknown modifier" error. So first tip: TURN ERROR REPORTING ON!

To fix it, try using a different delimiter, e.g. {}, as it's easier to read than loads of leaning toothpicks...

{^http(s)?://(([a-z]+)\.)*stackoverflow.com}

The other problem is the dot in the $domain becomes a wildcard match - anytime you insert unknown data into a regex, get in the habit of using preg_quote to escape it, e.g.

$pattern = "{^http(s)?://(([a-z]+)\.)*" . preg_quote($domain, '{') . "}";

(Note - nice catch from stema in the comments: if you use a different delimiter, you must pass that preg_quote. It's clever enough to spot paired delimiters, so if you pass { it will also escape }.)