gh-152056: Compile single-category character sets to a bare CATEGORY opcode (GH-152057)

A character set containing exactly one category, e.g. [\d] or [^\s], now
compiles to a single CATEGORY opcode (like \d or \S) instead of an IN
block.  The negated form maps to the complementary category.  This speeds
up matching and reduces the size of the compiled byte code.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: eendebakpt

Doc/whatsnew/3.16.rst

@@ -288,10 +288,12 @@ re
 --
 
 * Character class escapes (``\d``, ``\D``, ``\s``, ``\S``, ``\w`` and ``\W``)
-  outside a character set are now compiled to a single ``CATEGORY`` opcode
-  instead of being wrapped in an ``IN`` block.  This speeds up matching of
-  patterns such as ``\d+`` and reduces the size of the compiled byte code.
-  (Contributed by Serhiy Storchaka in :gh:`152033`.)
+  outside a character set, and character sets containing a single such escape
+  (such as ``[\d]`` or ``[^\s]``), are now compiled to a single ``CATEGORY``
+  opcode instead of being wrapped in an ``IN`` block.  This speeds up matching
+  of patterns such as ``\d+`` and reduces the size of the compiled byte code.
+  (Contributed by Serhiy Storchaka in :gh:`152033` and Pieter Eendebak in
+  :gh:`152056`.)
 
 module_name
 -----------

Lib/re/_parser.py

@@ -625,6 +625,12 @@ def _parse(source, state, verbose, nested, first=False):
                     subpatternappend((NOT_LITERAL, set[0][1]))
                 else:
                     subpatternappend(set[0])
+            elif _len(set) == 1 and set[0][0] is CATEGORY:
+                # optimization: a lone category like [\d] or [^\d]
+                if negate:
+                    subpatternappend((CATEGORY, CH_NEGATE[set[0][1]]))
+                else:
+                    subpatternappend(set[0])
             else:
                 if negate:
                     set.insert(0, (NEGATE, None))

Misc/NEWS.d/next/Library/2026-06-24-10-30-00.gh-issue-152056.Qk7mZ2.rst

@@ -0,0 +1,5 @@
+Optimize matching of a character set that contains a single character
+category, such as ``[\d]`` or ``[^\s]``: it is now compiled to a single
+``CATEGORY`` opcode, the same as the corresponding ``\d`` or ``\S`` escape,
+instead of being wrapped in an ``IN`` block.  This speeds up matching and
+reduces the size of the compiled byte code.  Patch by Pieter Eendebak.