Question: Say I get a patch created with `git format-patch`. The file is basically a unified diff with some metadata. If I open the file in Vim, I can see which lines have been modified, but I cannot see which *characters* in the changed lines differ. Does anyone know a way (in Vim, or some other free software that runs on Ubuntu) to visualize per-character differences?

A counter example where per-character diff is visualized is when executing `vimdiff a b`.

**update Fri Nov 12 22:36:23 UTC 2010**

[diffpatch is helpful](https://stackoverflow.com/questions/242646/use-vimdiff-with-a-diff-file/244766#244766) for the scenario where you're working with a single file.

**update Thu Jun 16 17:56:10 UTC 2016**

Check out [diff-highlight in git 2.9](https://github.com/blog/2188-git-2-9-has-been-released). This script does *exactly* what I was originally seeking.

Answer: 
In git, you can merge without committing. Merge your patch first, then do:

```
git diff --word-diff-regex=.

```

Note the dot after the equals sign.
Here are some versions with less noisy output than `git diff --word-diff-regex=<re>` and that require less typing than, but are equivalent to, `git diff --color-words --word-diff-regex=<re>`.

Simple (does highlight space changes):

```
git diff --color-words

```

Simple (highlights individual character changes; does not highlight space changes):

```
git diff --color-words=.

```

More complex (does highlight space changes):

```
git diff --color-words='[^[:space:]]|([[:alnum:]]|UTF_8_GUARD)+'

```

In general:

```
git diff --color-words=<re>

```

where `<re>` is a regexp defining "words" for the purpose of identifying changes.

These are less noisy in that they color the changed "words", whereas using just `--word-diff-regex=<re>` surrounds matched "words" with colored `-/+` markers.