preg_replace返回空字符串[duplicate]

This question already has an answer here:

Is there a PHP function that can escape regex patterns before they are applied? 2 answers
RegEx for word boundary but still match if is preceded or followed by special chars 1 answer

I am trying to highlight words in a text by using pregreplace. The words are within an array which I do feed into pregreplace. This used to work, but stopped maybe due to upgrading php in the past, now it returns nothing.

// Function highlights $words in $str 
function highlight_words($str, $words) {
    global  $color;
    if(is_array($words)) {
        foreach($words as $k => $word) {
            // $pattern[$k] = "~\b($word)\b~is";
            $pattern[$k] = "/$word/";
            $replace[$k] = '<span style="background: '.$color[2].';color:'.$color[4].';">\\1</span>';
        }
    }
    else {
        $pattern = "~\b($words)\b~is";
        $replace = '<span style="background: '.$color[2].';color:'.$color[4].';">\\1</span>';
    }
    return preg_replace($pattern,$replace ,$str);
}


echo highlight_words($text, $words);

values of pattern:

Array
(
[0] => /$18.5mUSD/
[1] => /$8,000,000.00/
[2] => /(at)/
[3] => /+43 688 649 45702/
...

values of $replace:

Array
(
[0] => <span style="background: #FF6B02;color:#FFFFFF;">\1</span>
[1] => <span style="background: #FF6B02;color:#FFFFFF;">\1</span>
[2] => <span style="background: #FF6B02;color:#FFFFFF;">\1</span>
[3] => <span style="background: #FF6B02;color:#FFFFFF;">\1</span>
...

preg_replace simply returns nothing. Seems to be a problem with array, but the syntax looks ok. If I change preg_replace to replace some simply regex pattern it will do, but it does not work with the array.

How can this be fixed?

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