annotation validation: validate regex in common name annotation (#10657)

* fix common name validation

* add tests

internal/ingress/annotations/parser/validators.go

@@ -117,6 +117,20 @@ func ValidateRegex(regex *regexp.Regexp, removeSpace bool) AnnotationValidator {
 	}
 }
 
+// CommonNameAnnotationValidator checks whether the annotation value starts with
+// 'CN=' and is followed by a valid regex.
+func CommonNameAnnotationValidator(s string) error {
+	if !strings.HasPrefix(s, "CN=") {
+		return fmt.Errorf("value %s is not a valid Common Name annotation: missing prefix 'CN='", s)
+	}
+
+	if _, err := regexp.Compile(s[3:]); err != nil {
+		return fmt.Errorf("value %s is not a valid regex: %w", s, err)
+	}
+
+	return nil
+}
+
 // ValidateOptions receives an array of valid options that can be the value of annotation.
 // If no valid option is found, it will return an error
 func ValidateOptions(options []string, caseSensitive, trimSpace bool) AnnotationValidator {