REPEAT
    statements
UNTIL expression

Marks the end of a REPEAT loop and performs the post-test that decides whether to loop again.
The block between REPEAT and UNTIL always runs at least once. After it runs, expression is evaluated:

If expression is true, the loop ends and execution continues after UNTIL.
If expression is false, control returns to the matching REPEAT.

Remarks: UNTIL must match the nearest preceding REPEAT and should appear on its own line.

Note: REPEAT…UNTIL blocks may be nested freely.

How to think about it

Read it as “do these statements until the condition becomes true” (exit-on-true).
If you want an entry-test loop instead, 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$) > 0
PRINT "Hello, "; NAME$

Wait until a key is pressed

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

Nested loops

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

Notes

The condition after UNTIL is an ordinary boolean expression; use comparisons like =, <>, <, >, <=, >=.
Because the test is after the body, the body runs once even if the condition is initially true.
REPEAT…UNTIL works alongside other control structures (FOR…NEXT, IF…ENDIF, WHILE…ENDWHILE) and may be nested within them.

See also:
REPEAT · WHILE · LEN · INKEY$