Commit: runtime(python): improve performance of number and ellipsis matching

[Christian Brabandt]

runtime(python): improve performance of number and ellipsis matching

Commit: https://github.com/vim/vim/commit/e9c5e56081d1923caab5efce1f6fca4e235cf350
Author: Julien Voisin <julien...@dustri.org>
Date: Tue Sep 1 18:54:45 2026 +0000

runtime(python): improve performance of number and ellipsis matching

Problem: The floating-point `.d` number match and the two ellipsis
matches are slow: each begins with a look-behind, which cannot
be reduced to a fixed first character, so the automatic regexp
engine selects the slower NFA backend for them.
Solution: Force the backtracking engine with \%#=1 on those three
patterns; it evaluates the look-behind far more efficiently.
Highlighting is unchanged.

Measured with :syntime over a 40000 line corpus: the three affected
rules drop from ~0.13s to ~0.008s (about -93%), which cuts the total
syntax parse cost by ~10% (1.85s to 1.65s).

closes: #21194

Signed-off-by: Julien Voisin <julien...@dustri.org>
Signed-off-by: Christian Brabandt <c...@256bit.org>

diff --git a/runtime/syntax/python.vim b/runtime/syntax/python.vim
index c66fee665..55c629d56 100644
--- a/runtime/syntax/python.vim
+++ b/runtime/syntax/python.vim
@@ -7,6 +7,7 @@
" 2026 Jan 26 by Vim Project: highlight constants #18922
" 2026 Mar 11 by Vim Project: fix number performance #19630
" 2026 May 27 by Vim Project: highlight `lazy` soft keyword (PEP 810) #20342
+" 2026 Aug 30 by Vim Project: improve number and ellipsis performance
" Credits: Neil Schemenauer <n...@python.ca>
" Dmitry Vasiliev
" Rob B
@@ -285,9 +286,9 @@ if !exists("python_no_number_highlight")
" \d\.\d
syn match pythonNumber
\ "\<\d\+\%(_\d\+\)*\.\d\+\%(_\d\+\)*\%([eE][+-]\=\d\+\%(_\d\+\)*\)\=[jJ]\=\>"
- " \.\d
+ " \.\d -- \%#=1 selects the faster backtracking engine (leading look-behind)
syn match pythonNumber
- \ "\%(^\|\W\)\@1<=\.\d\+\%(_\d\+\)*\%([eE][+-]\=\d\+\%(_\d\+\)*\)\=[jJ]\=\>"
+ \ "\%#=1\%(^\|\W\)\@1<=\.\d\+\%(_\d\+\)*\%([eE][+-]\=\d\+\%(_\d\+\)*\)\=[jJ]\=\>"
endif

" Group the built-ins in the order in the 'Python Library Reference' for
@@ -330,7 +331,8 @@ if !exists("python_no_builtin_highlight")
\ contains=ALLBUT,pythonBuiltin,pythonClass,pythonFunction,pythonType,pythonAsync
\ transparent
" the ellipsis literal `...` can be used in multiple syntactic contexts
- syn match pythonEllipsis "\.\@1<!\.\.\.\ze\.\@!" display
+ " \%#=1 selects the faster backtracking engine (leading look-behind)
+ syn match pythonEllipsis "\%#=1\.\@1<!\.\.\.\ze\.\@!" display
endif

" From the 'Python Library Reference' class hierarchy at the bottom.
@@ -390,7 +392,8 @@ if !exists("python_no_doctest_highlight")
syn region pythonDoctestValue
\ start=+^\s*\%(>>>\s\|\.\.\.\s\|"""\|'''\)\@!\S\++ end="$"
\ contained contains=pythonEllipsis
- syn match pythonEllipsis "\%(^\s*\)\@<!\.\@1<!\zs\.\.\.\ze\.\@!" display
+ " \%#=1 selects the faster backtracking engine (leading look-behind)
+ syn match pythonEllipsis "\%#=1\%(^\s*\)\@<!\.\@1<!\zs\.\.\.\ze\.\@!" display
\ contained containedin=pythonDoctest
else
syn region pythonDoctest