new wordRe class - a word that holds a regExp

- a possible future replacement for keyType, but the immediate use is the
    wordReList for grepping through other lists.
  - note that the argList treatment of '(' ... ')' yields quoted strings,
    which we can use for building a wordReList

minor cleanup of regExp class

  - constructor from std::string, match std::string and
    operator=(std::string&)
    rely on automatic conversion to Foam::string
  - ditch partialMatch with sub-groups, it doesn't make much sense

applications/test/string/stringTest.C

@@ -40,6 +40,25 @@ int main(int argc, char *argv[])
 
     Info<< test << endl;
 
+    // test sub-strings via iterators
+    string::const_iterator iter = test.end();
+    string::const_iterator iter2 = test.end();
+    string::size_type fnd = test.find('\\');
+
+    if (fnd != string::npos)
+    {
+        iter  = test.begin() + fnd;
+        iter2 = iter + 6;
+    }
+
+    Info<< "sub-string via iterators : >";
+    while (iter != iter2)
+    {
+        Info<< *iter;
+        iter++;
+    }
+    Info<< "<\n";
+
     Info<< string(test).replace("kj", "zzz") << endl;
     Info<< string(test).replace("kj", "") << endl;
     Info<< string(test).replaceAll("kj", "zzz") << endl;