Backreference

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.

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.