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php – Regex for matching CSS hex colors

Posted by: admin April 23, 2020 Leave a comment

Questions:

I’m trying to write regex that extracts all hex colors from CSS code.

This is what I have now:

Code:

$css = <<<CSS

/* Do not match me: #abcdefgh; I am longer than needed. */

.foo
{
    color: #cccaaa; background-color:#ababab;
}

#bar
{
    background-color:#123456
}
CSS;

preg_match_all('/#(?:[0-9a-fA-F]{6})/', $css, $matches);

Output:

Array
(
    [0] => Array
        (
            [0] => #abcdef
            [1] => #cccaaa
            [2] => #ababab
            [3] => #123456
        )

)

I don’t know how to specify that only those colors are matched which ends with punctuation, whitespace or newline.

How to&Answers:

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

Answer:

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

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

Answer:

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})

Answer:

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

Answer:

I’m not entirely sure if I got this right, but if you only want to match hex colors at the end of a CSS line:

preg_match_all('/#(?:[0-9a-fA-F]{6}|[0-9a-fA-F]{3})[\s;]*\n/',$css,$matches);

should work, all I did was add the optional \s; char group (optional semi-colon and spaces) and a line-break character (not optional) and it seemed to work.
And as @GolezTrol pointed out #FFF; is valid, too.

When tested on this:

$css = '/* Do not match me: #abcdefgh; I am longer than needed. */
.foo
{
    color: #CAB;
    background-color:#ababab;
}';
preg_match_all('/#(?:[0-9a-fA-F]{6}|[0-9a-fA-F]{3})[\s;]*\n/',$css,$matches);
var_dump($matches);

The output was:

array (array('#CAB;','#ababab;'))