string range string first last
Returns a range of consecutive characters from string, starting with the character whose index is first and ending with the character whose index is last. An index of 0 refers to the first character of the string. first and last may be specified as for the index method. If first is less than zero then it is treated as if it were zero, and if last is greater than or equal to the length of the string then it is treated as if it were end. If first is greater than last then an empty string is returned.
See also:
Tcl syntax help - Category Command - Category String Processing