diff --git a/src/emitterutils.cpp b/src/emitterutils.cpp
index d5a72cc..34da1bc 100644
--- a/src/emitterutils.cpp
+++ b/src/emitterutils.cpp
@@ -175,11 +175,17 @@ bool IsValidPlainScalar(const std::string& str, FlowType::value flowType,
   }
 
   // then check until something is disallowed
-  const RegEx& disallowed = (flowType == FlowType::Flow ? Exp::EndScalarInFlow()
-                                                        : Exp::EndScalar()) ||
-                            (Exp::BlankOrBreak() + Exp::Comment()) ||
-                            Exp::NotPrintable() || Exp::Utf8_ByteOrderMark() ||
-                            Exp::Break() || Exp::Tab();
+  static const RegEx& disallowed_flow =
+      Exp::EndScalarInFlow() || (Exp::BlankOrBreak() + Exp::Comment()) ||
+      Exp::NotPrintable() || Exp::Utf8_ByteOrderMark() || Exp::Break() ||
+      Exp::Tab();
+  static const RegEx& disallowed_block =
+      Exp::EndScalar() || (Exp::BlankOrBreak() + Exp::Comment()) ||
+      Exp::NotPrintable() || Exp::Utf8_ByteOrderMark() || Exp::Break() ||
+      Exp::Tab();
+  const RegEx& disallowed =
+      flowType == FlowType::Flow ? disallowed_flow : disallowed_block;
+
   StringCharSource buffer(str.c_str(), str.size());
   while (buffer) {
     if (disallowed.Matches(buffer)) {