|str||The string to convert to letters.|
With this function you can remove all characters that are not
classed as letters. This is handy for preventing people from
entering unwanted characters into a text entry (like entering "#"
to force a new line).
Note: This function only detects the 26 letter english alphabet from A - Z.
username = string_letters(username);
The above code will set the "username" variable to only hold the letter characters of the original string.