extract substring

substr extracts a substring of length len from the string str, starting at the specified offset. In this version, the first character position has offset 1. (In Acklam's original code, the first character position has offset 0, but this is inconsistent with Matlab conventions). If offset is negative, the position is reckoned by counting backwards from the end of the string. If len is omitted, substr returns everything to the end of the string. If len is negative, substr removes -len characters from the end of the string.