SUBSTR - extract substring from string.

SubString (S1,I1[,I2]) returns S

extracts a substring from S1, starting at I1 and containing I2 characters. The character at the beginning of S1 has an index of 0. If I1 is negative, the substring is taken I1 characters from the end of S1. If I2 is omitted or is -1, the substring extends from position I1 to the end of the string.