diff --git a/src/Lexer.cpp b/src/Lexer.cpp
index 0f53354ed..99a22cb9b 100644
--- a/src/Lexer.cpp
+++ b/src/Lexer.cpp
@@ -385,6 +385,28 @@ int Lexer::commonLength (const std::string& left, const std::string& right)
   return (int) l;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// Compares two strings with offsets, and returns the number bytes in common.
+//
+// left:   wonderful
+// l:      ^
+// right:  prowonderbread
+// r:         ^
+// returns:        ^ 6
+int Lexer::commonLength (
+  const std::string& left,
+  std::string::size_type l,
+  const std::string& right,
+  std::string::size_type r)
+{
+  while (left[l] == right[r]        &&
+         utf8_next_char (left,  l)  &&
+         utf8_next_char (right, r))
+    ;
+
+  return (int) l;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Lexer::Type::string
 //   '|"
diff --git a/src/Lexer.h b/src/Lexer.h
index f6db6e168..742dedd54 100644
--- a/src/Lexer.h
+++ b/src/Lexer.h
@@ -86,6 +86,7 @@ public:
   static int hexToInt               (int, int);
   static int hexToInt               (int, int, int, int);
   static int commonLength           (const std::string&, const std::string&);
+  static int commonLength           (const std::string&, std::string::size_type, const std::string&, std::string::size_type);
 
   bool isEOS                        () const;