REPEAT
    statements
UNTIL expression

Starts a post-test loop. The block between REPEAT and UNTIL executes at least once.
After the block runs, expression is evaluated; if it is true, the loop ends; if false, execution repeats from REPEAT.

Note: REPEAT pairs with the nearest following UNTIL.; UNTIL must appear on its own line.; You may nest REPEAT…UNTIL loops.

How to read it

Think of it as “do these statements **until the condition becomes true”** (exit-on-true).
If you prefer an entry-test loop (continue-on-true), use WHILE … ENDWHILE.

Examples

Count until a limit

N = 0
REPEAT
    PRINT "N = "; N
    N = N + 1
UNTIL N >= 5

Prompt until a non-empty string

NAME$ = ""
REPEAT
    PRINT "Enter your name:";
    INPUT NAME$
UNTIL LEN(NAME$) > 0
PRINT "Hello, "; NAME$

Nested REPEAT loops

Y = 0
REPEAT
    X = 0
    REPEAT
        PRINT "X="; X; ", Y="; Y
        X = X + 1
    UNTIL X = 3
    Y = Y + 1
UNTIL Y = 2

Loop until a key is pressed

PRINT "Press any key to stop."
REPEAT
    REM do some periodic work here
UNTIL INKEY$ <> ""

Notes

The condition on UNTIL is a normal boolean expression; use comparisons like =, <>, <, >, <=, >=.
Because the test occurs after the body, the body runs once even if the condition is initially true.
You can mix REPEAT…UNTIL with other control structures (e.g. place one inside a FOR loop) as needed.

See also:
WHILE · ENDIF · INPUT · LEN · INKEY$