Original Enhancement

Let a$ be a string expression,m, n be numeric expressions below.

Functions whose result is a string

SUBSTR$(a$,m,n) Substring of a$ from m-th character to n-th character.
MID$(a$,m,n) n characters of a$ from m-th character.
LEFT$(a$,n) The first n characters of a$
RIGHT$(a$,n) The last n characters of a$

The above functions are defined as follows.

DEF SUBSTR$(a$,m,n)=a$(m:n)
DEF MID$(a$,m,n)=a$(m:m+n-1)
DEF LEFT$(a$,n)=a$(1:n)
DEF RIGHT$(a$,n)=a$(LEN(a$)-n+1:LEN(a$))

Functions whose result is numeric.

BLEN(a$) The length of a$ in bytes.
This function is prepared for handling multi-byte characters.