Only fail on invalid \k back references, allowing things like '\'' and '\<' to pass.

Author: dop251

regexp_test.go

@@ -808,6 +808,18 @@ func TestECMANamedGroup(t *testing.T) {
 	if err == nil {
 		t.Fatal("Expected error")
 	}
+
+	re = MustCompile(`\'|\<?`, 0)
+	if m, err := re.MatchString("'"); err != nil {
+		t.Fatal(err)
+	} else if !m {
+		t.Fatal("Expected match")
+	}
+	if m, err := re.MatchString("<"); err != nil {
+		t.Fatal(err)
+	} else if !m {
+		t.Fatal("Expected match")
+	}
 }
 
 func TestECMAInvalidEscapeCharClass(t *testing.T) {

syntax/parser.go

@@ -1186,6 +1186,7 @@ func (p *parser) scanBasicBackslash(scanOnly bool) (*regexNode, error) {
 		return nil, p.getErr(ErrIllegalEndEscape)
 	}
 	angled := false
+	k := false
 	close := '\x00'
 
 	backpos := p.textpos()
@@ -1217,6 +1218,7 @@ func (p *parser) scanBasicBackslash(scanOnly bool) (*regexNode, error) {
 		}
 
 		ch = p.rightChar(0)
+		k = true
 
 	} else if !p.useOptionE() && (ch == '<' || ch == '\'') && p.charsRight() > 1 { // Note angle without \g
 		angled = true
@@ -1275,7 +1277,9 @@ func (p *parser) scanBasicBackslash(scanOnly bool) (*regexNode, error) {
 			}
 			return nil, p.getErr(ErrUndefinedNameRef, capname)
 		} else {
-			return nil, p.getErr(ErrMalformedNameRef)
+			if k {
+				return nil, p.getErr(ErrMalformedNameRef)
+			}
 		}
 	}