How do you state a regex pattern in Haskell?

I'm trying to do a regex replace with the following code

import Text.RE.Replace
import Text.RE.TDFA.String

onlyLetters :: String -> String
onlyLetters s = replaceAll "" $ s *=~ [re|$([^a-zA-Z])|]

I found it really hard to find any comprehensible documentation on this. This produces the compile error:

    src\Pangram.hs:6:53: error: parse error on input `]'
  |
6 | onlyLetters s = replaceAll "" $ (s *=~ [re|[a-zA-Z]|])
  |                                                     ^

Progress 1/2

--  While building package pangram-2.0.0.12 (scroll up to its section to see the error) using:
      C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_3.0.1.0_ghc-8.8.4.exe --builddir=.stack-work\dist\29cc6475 build lib:pangram test:test --ghc-options " -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1
PS C:\Users\mcleg\Exercism\haskell\pangram> stack test
pangram> configure (lib + test)
Configuring pangram-2.0.0.12...
pangram> build (lib + test)
Preprocessing library for pangram-2.0.0.12..
Building library for pangram-2.0.0.12..
[1 of 2] Compiling Pangram

src\Pangram.hs:7:56: error: parse error on input `]'
  |
7 | onlyLetters s = replaceAll "" $ s *=~ [re|$([^a-zA-Z])|]
  |                                                        ^

Progress 1/2

--  While building package pangram-2.0.0.12 (scroll up to its section to see the error) using:
      C:\sr\setup-exe-cache\x86_64-windows\Cabal-simple_Z6RU0evB_3.0.1.0_ghc-8.8.4.exe --builddir=.stack-work\dist\29cc6475 build lib:pangram test:test --ghc-options " -fdiagnostics-color=always"
    Process exited with code: ExitFailure 1

What is the problem with that bracket and how would I do this correctly? Thank you -Skye