Omission of the low index defaults to 0:
s = "12345"; s[:3] == "123"
Omission of the high index defaults to length of the string:
s = "12345"; s[3:] == "45"
Slice ranges were not correctly determined for negative indices and also
off by one in general (included one more element at the end of the
substring than what actually matched the index range).
It's now equivalent to Python slice notation. Accessing a string at
a single index is also the same as Python except that an out-of-range
index returns an empty string instead of throwing an expection.
The index expression can take up to two indices for the start and end
index of the substring to return (e.g. "mystring[1,3]"). Negative
indices are allowed, with -1 representing the last character in the
string. The indexing is not cyclic -- if the starting index is >= the
length of the string an empty string is returned, and if the ending
index is >= the length of the string then it's interpreted as the last
index of the string. Assigning to substrings accessed like this isn't
allowed.
