Regex for matching CSS hex colors

Despite this question's age I'd like to ammend the following:

^#([[:xdigit:]]{3}){1,2}$, where [[:xdigit:]] is a shorthand for [a-fA-F0-9].

So:
<?php preg_match_all("/^#(?>[[:xdigit:]]{3}){1,2}$/", $css, $matches) ?>

Also noteworthy here is the usage of a non-capturing group (?>...), to ensure we don't store data in memory we never wanted to store in the first place.

Try it online

Since a hex color code may also consist of 3 characters, you can define a mandatory group and an optional group of letters and digits, so the long and elaborate notation would be:

/#([a-f]|[A-F]|[0-9]){3}(([a-f]|[A-F]|[0-9]){3})?\b/

Or if you want a nice and short version, you can say that you want either 1 or 2 groups of 3 alphanumeric characters, and that they should be matched case insensitively (/i).

/#([a-f0-9]{3}){1,2}\b/i

Instead of [a-f0-9] you can also write [[:xdigit:]], if the regex engine supports this posix character class. In this case you can skip the /i at the end, and the whole formula is only two characters more, but arguably more descriptive.

/#([[:xdigit:]]{3}){1,2}\b/

The accepted answer shows you how to do it with regex, because that was your question. But you really don't need to use regex for this. Normally this is how I would do it:

if(ctype_xdigit($color) && strlen($color)==6){
    // yay, it's a hex color!
}

for 100.000 iterations:

Regex solution *: 0.0802619457245 seconds

Xdigit with strlen: 0.0277080535889 seconds

*: hex: ([a-fA-F0-9]{6})

Shorter version of GolezTrol's answer that avoids writing the character set twice:

/#([a-fA-F0-9]{3}){1,2}\b/