Returns the charIndex 'th character of the string argument. A charIndex of 0 corresponds to the first character of the string. charIndex may be specified as follows:
If charIndex is less than 0 or greater than or equal to the length of the string then an empty string is returned.
Returns for index an integer, the charIndex'th character in string. If index is end, the last character in string, and if index is end-integer, the last character minus the specified offset. (From the Tcl/Tk Reference Guide)
For example,
string index "abcde" "3"
returns "d" . Notice that the index is zero based - the first character is index 0.
string index "abcde" "10"
returns "" . There is no 10th character. No error is returned here.