XDocument replace all node value with lower case

>>> ', '.join('!+{}+!'.format(x) if x.lower()==w else x for x in s.split(', '))
'!+abc+!, !+Abc+!, !+aBc+!, abc-def, abca'

', '.join(['!+{}+!'.format(x) if x.lower()==w else x for x in s.split(', ')])

([A-Z]{2,})

\L$1

(\b[A-Z]{2,}\b)

var converted = str.Select(x => char.IsUpper(x) ? char.ToLower(x) : char.ToUpper(x));
var result = new string(converted.ToArray);

foreach (var c in str)
   if (char.IsUpper(c))
      result += char.ToLower(c);
   else 
      result += char.ToUpper(c);

foreach (var c in str)
    result += char.IsUpper(c) ? char.ToLower(c) : char.ToUpper(c);

for (int i = 0; i < str.Length; i++)
   if (char.IsUpper(str[i]))
      result += char.ToLower(str[i]);
   else 
      result += char.ToUpper(str[i]);

for (var i = 0; i < str.Length; i++)
     result += char.IsUpper(str[i]) ? char.ToLower(str[i]) : char.ToUpper(str[i]);

fixed (char* cStr = str)
   for (var c = cStr; c < cStr + str.Length; c++)
     *c = (char)(*c >= 'A' && *c <= 'Z' ? *c + 32 : *c >= 'a' && *c <= 'z' ? *c - 32 : *c);