Is it really required to escape backslashes in regex patterns?

Question

This might be a dumb question, but I have trouble understanding why the following code works as expected

$text = "ab    cd";
$text = preg_replace("/\s+/", "", $text);
echo $text;

and outputs abcd.

Shouldn't the backslash in \s be escaped to get its literal meaning inside the regular expression?

Answer 1

Not necessarily, because the string literal rules say that if \ is followed by anything other than another \ or a ' it is treated as any other character. This general rule also affects double-quoted strings, although in that case there are more recognized escape sequences than just these two.

You could escape it if you wanted to, but personally I think the world has enough backslashes already.