diff --git a/src/Lexer.cpp b/src/Lexer.cpp
index e11d20ac1..c09e88b99 100644
--- a/src/Lexer.cpp
+++ b/src/Lexer.cpp
@@ -198,6 +198,13 @@ bool Lexer::isWhitespace (int c)
           c == 0x3000);    // ideographic space Common  Separator, space
 }
 
+////////////////////////////////////////////////////////////////////////////////
+bool Lexer::isAlpha (int c)
+{
+  return (c >= 'A' && c <= 'Z') ||
+         (c >= 'a' && c <= 'z');
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Digits 0-9.
 bool Lexer::isDigit (int c)
@@ -279,7 +286,7 @@ bool Lexer::isBoundary (int left, int right)
   if (right == '\0')                                 return true;
 
   // XOR
-  if (isalpha (left)       != isalpha (right))       return true;
+  if (isAlpha (left)       != isAlpha (right))       return true;
   if (isDigit (left)       != isDigit (right))       return true;
   if (isWhitespace (left)  != isWhitespace (right))  return true;
 
diff --git a/src/Lexer.h b/src/Lexer.h
index 0258f29f8..afd7a0dc5 100644
--- a/src/Lexer.h
+++ b/src/Lexer.h
@@ -64,6 +64,7 @@ public:
   // Static helpers.
   static const std::string typeName (const Lexer::Type&);
   static bool isWhitespace         (int);
+  static bool isAlpha              (int);
   static bool isDigit              (int);
   static bool isHexDigit           (int);
   static bool isIdentifierStart    (int);