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A code rewrite tool for structural search and replace that supports ~every language.


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A short example below shows how comby simplifies matching and rewriting compared to regex approaches like sed.

Comby supports interactive review mode (click here to see it in action).

Install (pre-built binaries)

Mac OS X

  • brew install comby

Ubuntu Linux

  • bash <(curl -sL

  • Other Linux distributions: The PCRE library is dynamically linked in the Ubuntu binary. For other distributions like Arch Linux, a fixup is needed: sudo ln -s /usr/lib/ /usr/lib/ On Fedora, use sudo ln -s /usr/lib64/ /usr/lib64/ Alternatively, consider building from source.



  • docker pull comby/comby
click to expand an example invocation for the docker image

Running with docker on stdin:

docker run -a stdin -a stdout -a stderr -i comby/comby '(:[emoji] hi)' 'bye :[emoji]' lisp -stdin <<< '(👋 hi)'

Isn't a regex approach like sed good enough?

Sometimes, yes. But often, small changes and refactorings are complicated by nested expressions, comments, or strings. Consider the following C-like snippet. Say the challenge is to rewrite the two if conditions to the value 1. Can you write a regular expression that matches the contents of the two if condition expressions, and only those two? Feel free to share your pattern with @rvtond on Twitter.

if (fgets(line, 128, file_pointer) == Null) // 1) if (...) returns 0
      return 0;
if (scanf("%d) %d", &x, &y) == 2) // 2) if (scanf("%d) %d", &x, &y) == 2) returns 0
      return 0;

To match these with comby, all you need to write is if (:[condition]), and specify one flag that this language is C-like. The replacement is if (1). See the live example.

Build from source

  • Install opam. TL;DR do sh <(curl -sL

  • Run this if you don't have OCaml installed (it bootstraps the OCaml compiler):

opam init
opam switch create 4.11.0 4.11.0
  • Run eval $(opam env)

  • Install OS dependencies:

    • Linux: sudo apt-get install autoconf libpcre3-dev pkg-config zlib1g-dev m4 libgmp-dev libev4 libsqlite3-dev

    • Mac: brew install pkg-config gmp pcre libev

  • Then install the library dependencies:

git clone
cd comby 
opam install . --deps-only
  • Build and test
make test
  • Install comby on your PATH by running
make install