Speed up multiline regexes anchored by `^`.

Feature or enhancement

Proposal:

I noticed that for regexes of the form

regex = re.compile("^foo", re.MULTILINE)
regex.search(...)

there's a character by character loop calling SRE(match) every iteration. That's significantly slower than the regex

regex = re.compile("foo...")
regex.search(...)

which does a special prefix scan for "foo" without having to call SRE(match) on each character:

https://github.com/python/cpython/blob/42d645a7e81e0a5e6e0d35e222a8520450ac28ef/Modules/_sre/sre_lib.h#L1747

https://github.com/python/cpython/blob/42d645a7e81e0a5e6e0d35e222a8520450ac28ef/Modules/_sre/sre_lib.h#L1775-L1776

I would expect ^foo and foo to have more or less identical performance. A simple patch like this fixes that.

diff --git a/Modules/_sre/sre_lib.h b/Modules/_sre/sre_lib.h
index df377905bfa..70de4cccefd 100644
--- a/Modules/_sre/sre_lib.h
+++ b/Modules/_sre/sre_lib.h
@@ -1855,6 +1855,18 @@ SRE(search)(SRE_STATE* state, SRE_CODE* pattern)
             return 0;
         }
         while (status == 0 && ptr < end) {
+            if (pattern[0] == SRE_OP_AT &&
+                pattern[1] == SRE_AT_BEGINNING_LINE &&
+                (void*) ptr > state->beginning &&
+                !SRE_IS_LINEBREAK((int) ptr[-1]))
+            {
+                /* fast-forward to the next newline character */
+                while (ptr < end && !SRE_IS_LINEBREAK((int) *ptr)) {
+                    ptr++;
+                }
+                if (ptr >= end) {
+                    return 0;
+                }
+            }
             ptr++;
             RESET_CAPTURE_GROUP();
             TRACE(("|%p|%p|SEARCH\n", pattern, ptr));

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-148778
• gh-152339

先查看链接的 PR，然后阅读 Modules/_sre/sre_lib.h 中的 SRE 搜索循环，尤其是其中引用的前缀扫描代码。重现多行 ^foo 搜索和非锚定的 foo 搜索，并比较它们的性能。在不改变正则表达式行为的情况下，如果锚定情况的速度有可测量的接近，即视为完成。