public static Validator<CharSequence> letter(@NonNull final CharSequence errorMessage,
                                                 @NonNull final Case caseSensitivity,
                                                 final boolean allowSpaces,
                                                 @NonNull final char... allowedCharacters) {
        return new LetterValidator(errorMessage, caseSensitivity, allowSpaces, allowedCharacters);
    }
public static Validator < CharSequence > letter ( @ NonNull final CharSequence errorMessage , @ NonNull final Case caseSensitivity , final boolean allowSpaces , @ NonNull final char ... allowedCharacters ) { return new LetterValidator ( errorMessage , caseSensitivity , allowSpaces , allowedCharacters ) ; }
Creates and returns a validator, which allows to validate texts to ensure, that they only
contain letters. Letters are considered to be all alphabetical characters from A to B. It is
possible to specify, whether only uppercase or lowercase letters should be accepted, or if
they should be threatened case-insensitive. Additionally, it is possible to specify whether
spaces should be allowed and to add special characters, which should also be accepted. Empty
texts are also accepted.

@param errorMessage
The error message, which should be shown, if the validation fails, as an instance of
the type {@link CharSequence}. The error message may not be null
@param caseSensitivity
The case senstivitiy, which should be used by the validator, as a value of the enum
{@link Case}. The value may either be <code>UPPERCASE</code>, <code>LOWERCASE</code>
or <code>CASE_INSENSITIVE</code>
@param allowSpaces
True, if spaces should be allowed, false otherwise
@param allowedCharacters
The allowed special characters as an array of the type <code>char</code>. The array
may not be null
@return The validator, which has been created, as an instance of the type {@link Validator}