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2018, Oct 14 - Dimitri Merejkowsky
License: CC By 4.0

Introduction

Today I'd like to talk about a command-line tool I've been working on.

It's called ruplacer[1] and as the name suggest, it's *rually* cool and written in Rust.

1: https://github.com/dmerejkowsky/ruplacer

Basically, it finds and replaces text in source files. Here's a screenshot of ruplacer in action:

ruplacer screenshot [IMG]

Some nice features:

Skips files listed in `.gitignore`, as well as binary files
Runs in dry run mode by default (use `--go` to actually write the changes to the filesystem)
Defaults to searching the current working directory, although an other path maybe specified after the pattern and the replacement.
Uses Rust regular expressions, which means you can capture groups in the pattern and use them in the replacement. For instance:

# Replaces dates looking like DD/MM/YYYY to YYYY-MM-DD
$ ruplacer '(\d{2})/(\d{2})/(\d{4})' '$3-$1-$2'

Can be run with `--no-regex` if the pattern is just a substring and should not be used as a regular expression
Last, but not least there's also a `--subvert` mode, which allows you to perform replacements on a variety of case styles:

$ ruplacer --subvert foo_bar spam_eggs
Patching src/foo.txt
-- foo_bar, FooBar, and FOO_BAR!
++ spam_eggs, SpamEggs, and SPAM_EGGS!

How it works

Here's how it works:

First, we build a structopt[2] struct for the command-line arguments parsing. Depending on the presence of the `--subvert` or `--no-rexeg` flags, we build a *Query*, which can be of several types: `Substring`, `Regex` or `Subvert`.

2: https://crates.io/crates/structopt

Then we leverage the ignore[3] crate to walk through every file in the source directory while skipping files listed in `.gitignore`. By the way, the ignore crates comes directly from ripgrep[4], an awesome alternative to `grep` also written in Rust.

3: https://crates.io/crates/ignore

4: https://github.com/BurntSushi/ripgrep

Along the way, we build a *FilePatcher* from the source file and the query. The FilePatcher goes through every line of the file and then sends it along with the query to a *LinePatcher*.

The LinePatcher runs the code corresponding to the query type and returns a new string, using the Inflector[5] crate to perform case string conversions if required.

5: https://crates.io/crates/Inflector

Finally, if the string has changed, the FilePatcher builds a *Replacement* struct and pretty-prints it to the user. While doing so, it also keeps a record of the modified contents of the file. Finally, if not in dry-run mode, it overwrites the file with the new contents.

And that's pretty much it :)

Why I'm sharing this

The idea of ruplacer started almost a decade ago when a colleague of mine showed me a shell function called `replacer` (thanks, Cédric!) It was basically a mixture of calls to `find`, `sed` and `diff`. You can still find it online[6].

6: https://github.com/cgestes/ctafconf/blob/78b92a60bc185b73f95418e3e913e33aae8799f6/bin/replacer#L75

Because I wanted better cross-platform support, a dry-run mode and a colorful output, I rewrote it in Python[7] a few years ago. Along the way, the features, command line syntax and the style of the output changed quite a lot, but I've been using it regularly for all this time.

7: https://github.com/dmerejkowsky/replacer

Finally, after hearing about ripgrep and fd[8], I decided to give Rust a go, and that's how ruplacer, the third incarnation of this tool, was born. This makes me confident it's good enough for *you* to try.

8: https://github.com/sharkdp/fd

If you have `cargo` installed, you can get ruplacer by running `cargo install ruplacer`. Otherwise, you will find the source code[9] and pre-compiled binaries[10] on GitHub.

9: https://github.com/dmerejkowsky/ruplacer/tree/master/src

10: https://github.com/dmerejkowsky/ruplacer/releases

Cheers!

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