AT&T 6300 Programmer's Manual page 352

MID$
Function and Statement
As a function, MID$ returns a substring from
a specified string.
Syntax
string
start
length
Remarks
7-196
MID$ (string,start[,length))
the string from which the substring is taken
the character position of the beginning of the returned
string. It must be an integer expression whose value is >0.
the length of the returned string. It must be an integer
expression from 0 to 255.
The function MID$ returns a substring taken
from a specified string, starting from a
specified character position start. The
"length" of the substring taken can be
specified. If length is omitted or if there are
fewer than length characters to the right of
the specified character position, all
characters to the right of the specified
character position are returned. If length is
equal to zero, or if start is greater than the
length of string, then MID$ returns a null
string.
Also see LEFT$ and RIGHT$ functions in this
chapter.