Commodore BASIC has some interesting and simple string functions built in. Three of them are self explanatory: LEN, LEFT$ and RIGHT$. But others, like the mysterious MID$ and INSTR functions, are a little tricker, and I can never remember how they works.
So here’s a quick recap on how they all work.
LEN (A$)
Returns the length of any given string. For example,
a$=”the cake is a lie”
print len (a$)
17
returns 17, which is the number of characters in our string.
LEFT$ (A$,X)
The LEFT$ function takes the x left characters from a given string. Here’s an example:

a$="onetwothree" print left$(a$,3) one 
We get “one”, because those are the 3 leftmost characters in our string a$.
RIGHT$ (A$,X)
Likewise, RIGHT$ takes the x right characters from any given string:

a$="onetwothree" print right$(a$,5) three 
Here we get “three”, because those are the 5 right characters of a$.
MID$ (A$,X,Y)
MID$ is a little more complex. It takes x characters from a given string, starting at position y. Let’s look at our earlier example again:

a$="onetwothree" print mid$(a$,5,3) two 
We get “two”, because those are the 3 characters, starting at position 5. The first position in all these string operations counts as one rather than zero.
But did you know that MID$ can also be used to assign and replace different characters in a string? Consider this:

mid$(a$,5,3)="ten" print a$ onetenthree 
Now we’ve replaced the 3 characters in our string with another string, starting at position 5.
I had no idea it cold do that! All these string operations work in all variations of the Commodore BASIC, except for the MID$ assignment which only works on the Plus/4 and the C128.
INSTR (A$, B$)
On the Plus/4 and C128, we can even check if one string is contained in another and at which position this occurs. Consider this:

a$="the cake is a lie" b$="cake" print instr(a$,b$) 5 
In our example, INSTR returns 5 because “cake” has been found at position 5 of “the cake is a lie”.
We can also specify a position from which the search shall be started like this:
Now INSTR returns 0 because “cake” has not been found beyond position 6 of our input string.