substring

substring( 
S_string 
x_index 
[ x_length ] 
) 
=> t_result / nil

Creates a new substring from an input string, starting at an index point and continuing for a given length.

Creates a new substring from S_string with a starting point determined by x_index and length determined by an optional third argument x_length.

If S_string is a symbol, the substring is taken from its print name.
If x_length is not given, then all of the characters from x_index to the end of the string are returned.
If x_index is negative the substring begins at the indexed character from the end of the string.
If x_index is out of bounds (that is, its absolute value is greater than the length of S_string), nil is returned.

S_string	A string.
x_index	Starting point for returning a new string. Cannot be zero.
x_length	Length of string to be returned.

t_result	Substring of S_string starting at the character indexed by x_index, with a maximum of x_length characters.
`nil`	If x_index is out of bounds.

substring("abcdef" 2 4)

=> "bcde"
substring("abcdef" 4 2)

=> "de"
substring("abcdef" -4 2)

=> "cd"