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Summary: Haven't done a release in over a year, probably time for cargo update before publishing 0.4.4.

Reviewed By: zertosh

Differential Revision: D51724294

fbshipit-source-id: 00a32cf23595de71850b9439c339b1bf4fb787ca

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April 17, 2018 17:32
November 30, 2023 16:24
April 17, 2018 17:32


fastmod is a fast partial replacement for codemod. Like codemod, it is a tool to assist you with large-scale codebase refactors, and it supports most of codemod's options. fastmod's major philosophical difference from codemod is that it is focused on improving the use case "I want to use interactive mode to make sure my regex is correct, and then I want to apply the regex everywhere". For this use case, it offers much better performance than codemod. Accordingly, fastmod does not support codemod's --start, --end, or --count options, nor does it support anything like codemod's Python API.


Let's say you're deprecating your use of the <font> tag. From the command line, you might make progress by running:

fastmod -m -d /home/jrosenstein/www --extensions php,html \
    '<font *color="?(.*?)"?>(.*?)</font>' \
    '<span style="color: ${1};">${2}</span>'

For each match of the regex, you'll be shown a colored diff and asked if you want to accept the change (the replacement of the <font> tag with a <span> tag), reject it, or edit the line in question in your $EDITOR of choice.

NOTE: Whereas codemod uses Python regexes, fastmod uses the Rust regex crate, which supports a slightly different regex syntax and does not support look around or backreferences. In particular, use ${1} instead of \1 to get the contents of the first capture group, and use $$ to write a literal $ in the replacement string. See the regex crate's documentation for details.

A consequence of this syntax is that the use of single quotes instead of double quotes around the replacement text is important, because the bash shell itself cares about the $ character in double-quoted strings. If you must double-quote your input text, be careful to escape $ characters properly!

fastmod also offers a usability improvement over codemod: it accepts files or directories to process as extra positional arguments after the regex and substitution. For instance, the example above could have been rewritten as

fastmod -m --extensions php,html \
    '<font *color="?(.*?)"?>(.*?)</font>' \
    '<span style="color: ${1};">${2}</span>' \

This makes it possible to use fastmod to process a list of files from somewhere else if needed. Note, however, that fastmod does its own parallel directory traversal internally, so doing find ... | xargs fastmod ... may be much slower than using fastmod by itself.


fastmod is primarily supported on macOS and Linux.

fastmod has also been reported to work reasonably well on Windows. The major portability concerns are 1) the use of $EDITOR with a fallback and 2) the console UI, which is descended from codemod's ncurses-based text coloring & screen clearing code. Windows-specific issues and PRs will be considered as long as they aren't too invasive. For example, if something doesn't work on Windows because a Linux/Mac-specific API was used instead of equivalent POSIX or Rust standard library calls, we would be happy to fix that. On the other hand, we would like to avoid taking a direct winapi dependency or substantially increasing the size of our dependency graph for Windows-only enhancements.

Building fastmod

fastmod is written in (stable) Rust and compiles with Rust's cargo build system. To build:

$ git clone
$ cd fastmod
$ cargo build --release
$ ./target/release/fastmod --help

Installing fastmod

The easiest way to install fastmod is simply cargo install fastmod. If you have built fastmod from source following the directions above, you can install your build with cargo install.

How fastmod works

fastmod uses the ignore crate to walk the given directory hierarchy using multiple threads in parallel while respecting .gitignore. It uses the grep crate to match each file, reads matching files into memory, applies the given regex substitution one match at a time, and uses the diff crate to present the resulting changes as patches for human review.

Full documentation

See fastmod --help.


fastmod is Apache-2.0-licensed.