backreference

An item in a regular expression equivalent to the text matched by an earlier pattern in the expression.

The third part of the regular expression, \1, is the backreference and tells ColdFusion to refer back to the first subexpression[…]

Path traversal occurs when directory backreferences are used in a path to gain access to the parent folder of a subfolder.

To access (the text matched by an earlier pattern in a regular expression).

By wrapping it in parentheses, the attributes can be reused by backreferencing this part of the pattern[…]

It doesn't change what the part matches, so /\d+/ and /(\d+)/ will still match as many digits as possible, but in the latter case they will be remembered in a special variable to be backreferenced later.