Why special characters like = or " break PHP regexp when using \b word boundary?

Question

this is a follow up after reading How to specify "Space or end of string" and "space or start of string"?

From there, it states means to match a word

Answer 1

The problem is your use of \b which is a "word boundary." It's a placeholder for (^\w|\w$|\W\w|\w\W), where \w is a "word" character [A-Za-z0-9_] and \W is the opposite. The problem is that a " doesn't match the "word" characters, so the boundary condition is not met.

Try using a \s instead, which will match any whitespace character.

(?:^|\s)stackoverflow=""(?:\s|$)

Characters inside a class are not interpreted, except for ^ used as a negation operator at the beginning of a class, and - as a range operator. This is why [ ^] wouldn't work for you. It was searching for a literal ^.

$ php -a
Interactive shell

php > $input_line='
php ' stackoverflow="" xxx
php ' xxx stackoverflow="" xxx
php ' xxx stackoverflow=""
php ' ';
php > echo preg_replace('/(?:^|\s)stackoverflow=""(?:\s|$)/', 'OK', $input_line);
OKxxx
xxxOKxxx
xxxOK

https://regex101.com/r/nP2aB8/1