substring
The expression substring(
string,
start_index,
end_index)
yields the substring of string that starts at
start_index and ends at end_index, both inclusive. A
positive index counts from the string start: 1L
is the index of
the first character; a negative index counts from the string end:
1R
is the index of the last character. If end_index is
omitted, it is assumed to be the same as start_index, so
substring(
string,
index)
yields the character at
index in string. If end_index is less than
start_index, the function yields an empty string.