Fwd: git diff with “--word-diff-regex” extremely slow compared to “--word-diff”?

[Matthieu S <matthieu.stigler@gmail.com>]

Hi

When giving a custom regex to git diff --word-diff-regex= instead of
using the default --word-diff (which splits words on whitespace), git
slows down very considerably... I don't understand why such a speed
difference?

(this question was asked on stack overflow, but after two month
without answer, I'm asking it here instead. Post:
http://stackoverflow.com/questions/39027864/git-diff-with-word-diff-regex-extremely-slow-compared-to-word-diff).

Example (sorry, UNIX specific code): create two one-line files, and
two 200000-lines files:

echo aaa,bbb ,12,12,15 >file1.txt
echo aaa,bbb ,12,12,16 >file2.txt

awk '{for(i=0;i<200000;i++)print}' file1.txt > file1BIG.txt
awk '{for(i=0;i<200000;i++)print}' file2.txt > file2BIG.txt

Default --word-diff has no issues with the BIG files (cannot see time
difference):

git diff --word-diff file1.txt file2.txt
git diff --word-diff file1BIG.txt file2BIG.txt

Now use instead --word-diff-regex= argument (with regex from post:
http://stackoverflow.com/questions/10482773/also-use-comma-as-a-word-separator-in-diff
)

git diff --word-diff-regex=[^[:space:],] file1.txt file2.txt
git diff --word-diff-regex=[^[:space:],] file1BIG.txt file2BIG.txt

Why is the speed so different if one uses --word-diff instead of
--word-diff-regex= ? Is it just because my expression is (slightly)
more complex than the default one (split on period instead of only
whitespace) ? Or is it that the default word-diff is implemented
differently/more efficiently? How can I overcome this speed slowdown?

Thanks!!

Matthieu


PS: using git 2.7.4 on Ubuntu 16.04