The newer RFC (RFC 3986[1] from 2005, which I assume is being used here) for URIs doesn't specifically mention them in the spec. You have to go back to 1998 and rfc 2396[2].

RFC 3986 (circa 2005):

      path          = path-abempty    ; begins with "/" or is empty
                    / path-absolute   ; begins with "/" but not "//"
                    / path-noscheme   ; begins with a non-colon segment
                    / path-rootless   ; begins with a segment
                    / path-empty      ; zero characters

      path-abempty  = *( "/" segment )
      path-absolute = "/" [ segment-nz *( "/" segment ) ]
      path-noscheme = segment-nz-nc *( "/" segment )
      path-rootless = segment-nz *( "/" segment )
      path-empty    = 0<pchar>

      segment       = *pchar
      segment-nz    = 1*pchar
      segment-nz-nc = 1*( unreserved / pct-encoded / sub-delims / "@" )
                    ; non-zero-length segment without any colon ":"

      pchar         = unreserved / pct-encoded / sub-delims / ":" / "@"

RFC 2396 (circa 1998):

      path          = [ abs_path | opaque_part ]

      path_segments = segment *( "/" segment )
      segment       = *pchar *( ";" param )
      param         = *pchar

      pchar         = unreserved | escaped |
                      ":" | "@" | "&" | "=" | "+" | "$" | ","

IIRC, somewhere in RFC 3986 it alludes to the fact that you could still do something like that, but it would be scheme-specific, and not part of the URI spec.

Also of note is that pipes ("|") are no longer even mentioned in RFC 3986, but they were characterized as 'unwise' in 1998.

RFC 2396:

   unwise      = "{" | "}" | "|" | "\" | "^" | "[" | "]" | "`"

[1] http://www.apps.ietf.org/rfc/rfc3986.html#sec-3.3

[2] http://www.apps.ietf.org/rfc/rfc2396.html#sec-3.3