MASK_...() (STRING)

 Function
  Return a string for use as a valid character mask.

 Syntax
  MASK_...()   No arguments are required

  MASK_ should be followed by one of the following mask types:
        ALNUM, ALPHA, ASCII, FILE, NUM, PATH, or PWD.

 Return Type & Value
  STRING
  Returns a string with a set of characters to use as valid input for an
  INPUTSTR or PROMPTSTR statement.

 Remarks
  There are many situations in which you will need to use an INPUTSTR or
  PROMPTSTR statement to access the input field length of flags.  However,
  all you need to use a 'standard' set of input characters.  These
  functions provide you with some of the most common valid character
  masks.  They are:

  MASK_ALNUM()   A-Z, a-z, and 0-9;
  MASK_ALPHA()   A-Z and a-z;
  MASK_ASCII()   all characters from space (ASCII 32) to tilde (ASCII 126);
  MASK_FILE()    all legal file name characters;
  MASK_NUM()     0-9;
  MASK_PATH()    all legal path name characters; and, finally,
  MASK_PWD()     a set of valid characters for use in passwords.

 Examples
  INTEGER i
  STRING  s
  INPUTSTR "Enter a number from 0 to 1000",i,@X0E,4,MASK_NUM(),DEFS
  PROMPTSTR 148,s,12,MASK_PWD(),ECHODOTS
  INPUTSTR "Enter your comment",s,@X0E,60,MASK_ASCII(),DEFS