MySQL 8.0

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select version();


version()


8.0.22


CREATE TABLE BUCHUNGEN(
    BUCHUNGSTEXT varchar(30)
)
INSERT INTO BUCHUNGEN(BUCHUNGSTEXT) VALUES('RE12342020 Karl Mustermann');
INSERT INTO BUCHUNGEN(BUCHUNGSTEXT) VALUES('RE45672020 Grete Beispiel');
SELECT 
  REGEXP_SUBSTR(t.BUCHUNGSTEXT, 'RE[0-9]+') AS BELEGFELD, 
  BUCHUNGSTEXT
FROM BUCHUNGEN t


BELEGFELD
BUCHUNGSTEXT


RE12342020
RE12342020 Karl Mustermann

RE45672020
RE45672020 Grete Beispiel

BELEGFELD	BUCHUNGSTEXT
RE12342020	RE12342020 Karl Mustermann
RE45672020	RE45672020 Grete Beispiel