Substrings can be specified by appending a range specifier to any string,
string variable, or string-valued function. The range specifier has the
form [begin:end], where begin is the index of the first character of the
substring and end is the index of the last character of the substring.
The first character has index 1. The begin or end fields may be empty, or
contain '*', to indicate the true start or end of the original string.
E.g. str[:] and str[*:*] both describe the full string str.
