A character substring is a contiguous segment of a character
  variable, character array element, or character field reference.
  It has one of the following forms:

     v([e1]:[e2])
     a(s[,s]...)([e1]:[e2])

     v    Is a character variable name
     a    Is a character array name
     s    Is a subscript expression
     e1   Is a numeric expression specifying the leftmost
          character position of the substring
     e2   Is a numeric expression specifying the rightmost
          character position of the substring

  Both e1 and e2 must be within the range 1,2, ..., len, where len is
  the length of the parent character string.  If e1 exceeds e2, the
  substring has length zero.