so-37912233.elm

import Html exposing (..)
import Html.Attributes exposing (..)
import Combine exposing (..)
import Combine.Char exposing (..)
import Combine.Infix exposing (..)
import String
import List.Extra exposing (groupWhile)

testInput =
  "hello (!BOLD!) I Am a bold text (!BOLD!)bla bla la"

main =
  case parse htmlParser testInput of
    (Ok htmls, _) -> div [] htmls
    (Err err, _) -> div [ style [("color", "red")] ] [ text <| toString <| err]

type Style
  = Unstyled
  | Bold

htmlParser : Parser (List (Html msg))
htmlParser =
  styleParser False
    `andThen` (succeed << foldStyledHtml)

foldStyledHtml : List (Char, Style) -> List (Html msg)
foldStyledHtml chars =
  let
    foldSingleStyledHtml =
      List.foldr (\(c, s) (cs, _) -> (c :: cs, s)) ([], Unstyled)
        >> \(chars, style) ->
          let str = String.fromList chars
          in case style of
              Unstyled -> text str
              Bold -> b [] [ text str ]
  in
    groupWhile (\a b -> snd a == snd b) chars
      |> List.map foldSingleStyledHtml

styleParser : Bool -> Parser (List (Char, Style))
styleParser bolded =
  let
    style = if bolded then Bold else Unstyled
  in
    (end `andThen` always (succeed []))
      <|> (string "(!BOLD!)" `andThen` \_ -> styleParser (not bolded))
      <|> (anyChar
           `andThen` \c -> styleParser bolded
           `andThen` \cs -> (succeed ((c, style) :: cs)))