How can I visualize per-character differences in a unified diff file?

Question

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.

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

The above regex (from Thomas Rast) does a decent job of separating diff fragments at the punctuation/character level (while not being as noisy as --word-diff-regex=.).

I posted a screenshot of the resulting output here.

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Update:

This article has some great suggestions. Specifically, the contrib/ tree of the git repo has a diff-highlight perl script that shows fine-grained highlights.

Quick start to use it:

$ curl https://git.kernel.org/cgit/git/git.git/plain/contrib/diff-highlight/diff-highlight > diff-highlight
$ chmod u+x diff-highlight
$ git diff --color=always HEAD~10 | diff-highlight | less -R

Given your references to Vim in the question, I'm not sure if this is the answer you want :) but Emacs can do this. Open the file containing the diff, make sure that you're in diff-mode (if the file is named foo.diff or foo.patch this happens automatically; otherwise type M-x diff-mode RET), go to the hunk you are interested in and hit C-c C-b for refine-hunk. Or step through the file one hunk at a time with M-n; that will do the refining automatically.

If you have nothing against installing NodeJS, there's a package called "diff-so-fancy" (https://github.com/so-fancy/diff-so-fancy), which is very easy to install and works perfectly:

npm install -g diff-so-fancy
git diff --color | diff-so-fancy | less -R

Edit: Just found out it's actually a wrapper for the official diff-highlight... At least it's easier to install for perlophobes like me and the GitHub page is nicely documented :)

Am not aware of per character difference tool, but there is a per word difference tool: wdiff.

refer examples Top 4 File Difference Tools on UNIX / Linux – Diff, Colordiff, Wdiff, Vimdiff.