fast way to find the first letter in a richTextBox?

[ektoras]

In a very large richTextBox I want to find all letters.
Is there something faster than :

char[] letters={ 'a','b','c','d','e','f','g','h','i', 'j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S',
'T','U','V','W','X','Y','Z'} ;
richTextBox1.Text.IndexOfAny(letters, i);

OR

richTextBox1.Text[i].IsLetter;