You can use any alphanumeric characters for basic string matching purposes.
lower case 'a' through 'z'
upper case 'A' through 'Z'
numerals '0' though '9'
Thus, typing "confidential" and marking the test as Case-Insensitive denotes that either "Confidential" or "confidential" will match. Likewise, the string "007forever" marked as Case-Sensitive denotes a search for the exact phrase "007forever".