The SUBSTR function returns a substring (from a string specified
  as  a  parameter)  that  is  of the specified starting point and
  length.  The return value is compatible with  all  other  string
  types.

  Syntax:

     SUBSTR( str,start [,length] )

  The parameter 'str' is a character string value;  the  parameter
  'start' is an integer value that indicates the starting position
  of the substring.  The parameter 'length' is  an  integer  value
  that  indicates  the  length  of the substring.  If the 'length'
  argument is omitted, the length is computed to be the  remainder
  of  the  string  value  beginning at the starting position.  The
  'length' argument cannot be omitted on OpenVMS VAX systems.  For
  example,

     SUBSTR(string,start_index)

  is identical to

     SUBSTR(string,start_index,length(string)-string_index+1)

  When you use the SUBSTR function,  the  value  of  the  starting
  position must be greater than 0, the value of the length must be
  greater than or equal to 0, and there must be enough  characters
  following  the starting position to construct a substring of the
  specified length.