Set ValidChars to restrict the characters that can be validly entered as part of the text representation of the field's value. ValidChars is used by the IsValidChar method to determine whether a character typed into a data-aware edit control can make up part of the field's value. Unlike the EditMask property which specifies which characters can appear in specific positions, ValidChars limits the characters that can appear anywhere in the field's value. 

TField initializes ValidChars to include all characters. Numeric descendants of TField change the default value to include only the characters that can make up a numeric value. Applications can change this value to restrict the character set for a field in other ways or to allow characters that are not permitted by default. 

EditMask 

IsValidChar