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