diff --git a/src/Lexer.cpp b/src/Lexer.cpp
index d841b6b97..0f53354ed 100644
--- a/src/Lexer.cpp
+++ b/src/Lexer.cpp
@@ -367,6 +367,24 @@ int Lexer::hexToInt (int c0, int c1, int c2, int c3)
           hexToInt (c3);
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// Compares two strings, and returns the number bytes in common.
+//
+// left:   wonderful
+// right:  wonderbread
+// returns:     ^ 6
+int Lexer::commonLength (const std::string& left, const std::string& right)
+{
+  std::string::size_type l = 0;
+  std::string::size_type r = 0;
+  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 f92582131..f6db6e168 100644
--- a/src/Lexer.h
+++ b/src/Lexer.h
@@ -85,6 +85,7 @@ public:
   static int hexToInt               (int);
   static int hexToInt               (int, int);
   static int hexToInt               (int, int, int, int);
+  static int commonLength           (const std::string&, const std::string&);
 
   bool isEOS                        () const;