Question

The pattern below seems to work in regex editors, but it doesn’t work in PHP (no error).
I thought by adding delimiters and running the pattern through preg_quote would address this.
Would appreciate any help on what step I’m missing here.

Code sample:

$pattern = '%(?<=@address|.)singleline(?=[^\]\[]*\])%';  
$pattern = preg_quote($pattern);
$output  = preg_replace($pattern, "", $output);

HTML Sample:

  <p>[@address|singleline]</p>

Answer 1

preg_quote escapes characters that are regular expression syntax characters. These include . \ + * ? [ ^ ] $ ( ) { } = ! < > | : -. Try not using preg_quote.

$pattern = '%(?<=@address|.)singleline(?=[^\]\[]*\])%';  
$output  = preg_replace($pattern, "", $output);

EDIT:
You might want to use preg_quote if you had content you wanted to include in your regex pattern which contained characters used in regex syntax. For example:

$input = "item 1 -- total cost: $5.00";
$pattern = "/total cost: " . preg_quote("$5.00") . "/";
// $pattern should now be "/total cost: \$5.00/"
$output = preg_replace($pattern, 'five dollars', $input);

In this case, you need to escape the $ because it is used in the regex syntax. To search for it, your regex should use \$ instead of $. Using preg_quote performs this alteration for you.