ClearAll[foo, labeledFoo];

labeledFoo = {"FooBarBazQuux", foo};

labeledFoo /. hdr_String :>
  StringReplace[hdr, l_?LowerCaseQ ~~ U_?UpperCaseQ :> l <> " " <> U]

(* {"Foo Bar Baz Quux", foo} *)

labeledFoo /. hdr_String :>
  StringReplace[hdr, l_?LowerCaseQ ~~ U_?UpperCaseQ :> l <> " " <> U] /.
    {hdr_String, x_} :> (hdr -> x)

(* "Foo Bar Baz Quux" -> foo *)

labeledFoo /. {hdr_String, x_} :> (Rule @@ {StringReplace[hdr,
  l_?LowerCaseQ ~~ U_?UpperCaseQ :> l <> " " <> U], x})

(* "Foo Bar Baz Quux" -> foo *)

labeledFoo /. {hdr_String, x_} :> (StringReplace[hdr,
 l_?LowerCaseQ ~~ U_?UpperCaseQ :> l <> " " <> U] -> x)

(* "Fo" ~~ l <> " " <> U ~~ "a" ~~ l <> " " <> U ~~ "a" ~~
  l <> " " <> U ~~ "uux" -> foo *)

StringReplace["FooBarBazQuux",l$_?LowerCaseQ~~U$_?UpperCaseQ:>l<>" "<>U]

{{1, 2}, 3} /. {x_List, y_Integer} :>
  Rule[
     Replace[x, {l_Integer, u_Integer} :> l + u],
     y
  ]

(* l + u -> 3 *)

{x_, x+1} -> x^2

{{1, 2}, 3} /. {x_List, y_Integer} :>
  Block[{yy = y},
    Rule[
      Replace[x, {l_Integer, u_Integer} :> l + u],
      yy]
  ]

(* 3 -> 3 *)