From 9f55551f3df238e58315e724e50cb0d574d75b94 Mon Sep 17 00:00:00 2001
From: yannvgn <hi@yannvgn.io>
Date: Wed, 31 Jul 2019 20:50:39 +0200
Subject: [PATCH] bpo-37723: Fix performance regression on regular expression
 parsing. (GH-15030)

Improve performance of sre_parse._uniq function.
---
 Lib/sre_parse.py                                          | 8 +-------
 Misc/ACKS                                                 | 1 +
 .../next/Library/2019-07-31-16-49-01.bpo-37723.zq6tw8.rst | 2 ++
 3 files changed, 4 insertions(+), 7 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-07-31-16-49-01.bpo-37723.zq6tw8.rst

diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index 84c912573e7..83119168e63 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -430,13 +430,7 @@ def _escape(source, escape, state):
     raise source.error("bad escape %s" % escape, len(escape))
 
 def _uniq(items):
-    if len(set(items)) == len(items):
-        return items
-    newitems = []
-    for item in items:
-        if item not in newitems:
-            newitems.append(item)
-    return newitems
+    return list(dict.fromkeys(items))
 
 def _parse_sub(source, state, verbose, nested):
     # parse an alternation: a|b|c
diff --git a/Misc/ACKS b/Misc/ACKS
index 9ff74ec0cca..6463b535e17 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -1708,6 +1708,7 @@ Michael Urman
 Hector Urtubia
 Lukas Vacek
 Ville Vainio
+Yann Vaginay
 Andi Vajda
 Case Van Horsen
 John Mark Vandenberg
diff --git a/Misc/NEWS.d/next/Library/2019-07-31-16-49-01.bpo-37723.zq6tw8.rst b/Misc/NEWS.d/next/Library/2019-07-31-16-49-01.bpo-37723.zq6tw8.rst
new file mode 100644
index 00000000000..65507bd0dc9
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-07-31-16-49-01.bpo-37723.zq6tw8.rst
@@ -0,0 +1,2 @@
+Fix performance regression on regular expression parsing with huge
+character sets. Patch by Yann Vaginay.
\ No newline at end of file