Understand the XPath expression

Question

I want to understand the below XPath expression

fn:replace ($var,concat('^.*',fn:replace('.','(\.|$$|$$|\|\||\-|\^|\$|\?|\*|\+|\{|\}|$|$)','\$1')),'')

I understand
\ denotes an escape character
| denotes OR
. denotes any single character

But I am confused with the use of \$1 here. What is it referring to?

Robby Cornelissen · Answer

$1 is a back-reference.

The goal of the expression is to replace any regex meta-character (., [, ], and so on) with an escaped version of that character.