`b`i`o`u{I AM AN EXPERT PROGRAMMER}
`sup{NO `sup{MORE `sup{NESTING `sub{PROBLEMS`b`i{!!}}}}}Haskell is 30 lines.
module Parser where
import Text.ParserCombinators.Parsec
import Data.Maybe (fromJust)
delimiters = [('{','}'), ('(',')'), ('<','>'), ('[',']'), ('^','$')]
data Entity = Text String
| Tag String Entity
| Collection [Entity]
deriving (Show, Eq)
Collection xs `addChild` e = Collection $ xs ++ [e]
character resvd = ((char '\\' >> anyChar) <?> "escaped char") <|> noneOf ('`' : resvd)
tagChar = ((char '\\' >> anyChar) <?> "escaped char") <|> noneOf ('`' : map fst delimiters)
nested = do char '`'
tag <- many1 tagChar
value <- nested <|> block
return $ Tag tag value
block = do opening <- oneOf $ map fst delimiters
let closing = fromJust $ lookup opening delimiters
source (Collection []) (char closing) [closing]
text resvd = return . Text =<< many1 (character resvd)
source :: Entity -> Parser a -> [Char] -> Parser Entity
source coll fin resvd = (<|>)
(fin >> return coll)
(do e <- (nested <?> "nested tag") <|> (text resvd <?> "text")
source (coll `addChild` e) fin resvd)
full = source (Collection []) eof []
toBBCode (Collection xs) = concatMap toBBCode xs
toBBCode (Tag t v) = "[" ++ t ++ "]" ++ toBBCode v ++ "[/" ++ t ++ "]"
toBBCode (Text t) = t