gh-148762: Speed up multiline regexes anchored by ^

Misc/NEWS.d/next/Library/2026-04-19-23-29-38.gh-issue-148762.HSCJka.rst

@@ -0,0 +1,2 @@
+Multiline regexes starting with a caret, such as ``re.compile("^foo",
+re.MULTILINE)``, now run significantly faster.

Modules/_sre/sre_lib.h

@@ -1848,6 +1848,29 @@ SRE(search)(SRE_STATE* state, SRE_CODE* pattern)
             ptr++;
             RESET_CAPTURE_GROUP();
         }
+    } else if (pattern[0] == SRE_OP_AT &&
+               pattern[1] == SRE_AT_BEGINNING_LINE) {
+        /* pattern is anchored at the start of a line (MULTILINE "^").
+           Only the start of the string and the character after a linebreak
+           can match, so jump from one line start to the next instead of
+           trying SRE(match) at every position. */
+        end = (SRE_CHAR *)state->end;
+        TRACE(("|%p|%p|SEARCH AT_BEGINNING_LINE\n", pattern, ptr));
+        state->start = state->ptr = ptr;
+        status = SRE(match)(state, pattern, 1);
+        state->must_advance = 0;
+        while (status == 0) {
+            /* skip to the next linebreak ... */
+            while (ptr < end && !SRE_IS_LINEBREAK(*ptr))
+                ptr++;
+            if (ptr >= end)
+                return 0;
+            ptr++;  /* ... and step past it, onto a line start */
+            RESET_CAPTURE_GROUP();
+            TRACE(("|%p|%p|SEARCH AT_BEGINNING_LINE\n", pattern, ptr));
+            state->start = state->ptr = ptr;
+            status = SRE(match)(state, pattern, 0);
+        }
     } else {
         /* general case */
         assert(ptr <= end);