A string.
The position number of the first character of the substring.
The length of the substring.
This function returns a substring of the input string starting at the start_char position and running for length characters. The first character in the string is numbered zero. If start_char is greater than the length of the string, the function returns an empty string. If start_char is negative, it is indexed from the end of the string. If it is negative and greater than the length of the string, it is treated as zero--the beginning of the string.
If there are fewer characters than length in the string, or if length is -1, then the substring contains all characters from start_char to the end of the string.
Gamma> substr("Acme widgets",7,3);
"dge"
Gamma> substr("Acme widgets",9,-1);
"ets"
Gamma> substr("Acme widgets",-7,4);
"widg"
Gamma> substr("Acme widgets",-30,4);
"Acme"
Gamma>