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Changing <q> and </q> tags to " pairs in specific places
[caption id="attachment_100" align="aligncenter" width="300" caption="This is an image caption"]

import Data.List
import System.IO

main = do
   inh  <- openFile "input.txt"  ReadMode
   outh <- openFile "output.txt" WriteMode
   str <- hGetContents inh
   hPutStrLn outh (outsideCaption str)
   hClose inh
   hClose outh

outsideCaption::String->String
outsideCaption [] = []
outsideCaption str@(x:xs)
    | isPrefixOf "[caption" str = insideCaption str
    | otherwise                 = x:outsideCaption xs


insideCaption::String->String
insideCaption []       = []
insideCaption (']':xs) = ']':outsideCaption xs
insideCaption str@(x:xs)
    | (isPrefixOf "<q>"  str) = '"':insideCaption (drop 3 str)
    | (isPrefixOf "</q>" str) = '"':insideCaption (drop 4 str)
    |  otherwise              = x   :insideCaption         xs

main = interact outsideCaption

[rothesay]Ygfijj: echo "testing <q> [caption<q></q>]" | ./test
testing <q> [caption""]