Extract a substring from a string
String_Type substr (String_Type s, Integer_Type n, Integer_Type len)
The substr function returns a substring with length len
of the string s beginning at position n. If len is
-1, the entire length of the string s will be used for
len. The first character of s is given by n equal
to 1.
substr ("To be or not to be", 7, 5);
returns "or no"
The expression substr(s, n, 1) is equivalent to the array
index expression s[n-1].
is_substr, strlen