However if a create an escaped string directly like

var a = "\d+"

i get the correct output

\d+

(that i can further use to create a regex Patter) is this due to the way

String::replace

works? if so, isn't this a bug, why is it removing my escape sequences?

The reason is that the replacement can contain references to capture groups from the regex, using e.g.

$1

. Now to add a literal

$

in the replacement, you thus have to quote the

$

using

\

. And because this

\

quoting is not limited to just looking at a following

$

, any \ that you want in the output needs to be doubled. Details are in

Matcher::appendReplacement

description which is internally used for all of this.