string - An attribute of base type string, an expression that results in a string or a constant string value.
startIndex - A positive integer value indicating at which character the substring should start (inclusive). This can be either an attribute of base type integer, an expression that results in an integer or a constant integer value. The first character starts at index 0.
endIndex - A positive integer value indicating at which character the substring should end (exclusive). This can be either an attribute of base type integer, an expression that results in an integer or a constant integer value. The endIndex cannot exceed the length of the string and should be larger than the startIndex.