string sub ranging - comments by Grey@TMI-2

You can take a substring from a variable ( str ) buy using the substring
operation ( str[n1..n2] ).  Positive values are taken from the left and
negative values from the right.  If a value is greater than the length of
the string it will be treated as being equal to the length of the string.

If the two values are equal ( str[n1..n1] ) then the character at that
position ( n1 ) is returned.  If both values point to positions beyond
the same end of the string the null string ( "" ) is returned.  If the
position pointed to by the first value is after the one pointed to by
the second then the null string is also returned.

Examples:

   str = "abcdefg"

   str[0..0] ==       "a"
   str[0..-1] ==      "abcdefg"
   str[-4..-2] ==     "def"
   str[-7..6] ==      "abcdefg"
   str[3..2] ==       ""