2 files changed, 18 insertions, 1 deletions
diff --git a/src/regexp/syntax/prog.go b/src/regexp/syntax/prog.go
index 896cdc42c2..66995e2052 100644
--- a/src/regexp/syntax/prog.go
+++ b/src/regexp/syntax/prog.go
@@ -106,7 +106,9 @@ func EmptyOpContext(r1, r2 rune) EmptyOp {
 // during the evaluation of the \b and \B zero-width assertions.
 // These assertions are ASCII-only: the word characters are [A-Za-z0-9_].
 func IsWordChar(r rune) bool {
-	return 'A' <= r && r <= 'Z' || 'a' <= r && r <= 'z' || '0' <= r && r <= '9' || r == '_'
+	// Test for lowercase letters first, as these occur more
+	// frequently than uppercase letters in common cases.
+	return 'a' <= r && r <= 'z' || 'A' <= r && r <= 'Z' || '0' <= r && r <= '9' || r == '_'
 }
 
 // An Inst is a single instruction in a regular expression program.
diff --git a/src/regexp/syntax/prog_test.go b/src/regexp/syntax/prog_test.go
index 5603aea228..54dd1dd38d 100644
--- a/src/regexp/syntax/prog_test.go
+++ b/src/regexp/syntax/prog_test.go
@@ -127,3 +127,18 @@ func BenchmarkEmptyOpContext(b *testing.B) {
 		EmptyOpContext(r1, -1)
 	}
 }
+
+var sink any
+
+func BenchmarkIsWordChar(b *testing.B) {
+	const chars = "Don't communicate by sharing memory, share memory by communicating."
+	for i := 0; i < b.N; i++ {
+		for _, r := range chars {
+			sink = IsWordChar(r)
+		}
+	}
+	if sink == nil {
+		b.Fatal("Benchmark did not run")
+	}
+	sink = nil
+}