MySQL 8.0

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CREATE TABLE textlist
    (`text` varchar(17))
;
    
INSERT INTO textlist
    (`text`)
VALUES
    ('ab astest'),
    ('ab astestabc'),
    ('ab as'),
    ('ablxyx pqr tpl'),
    ('ab as ablmnr'),
    ('wasab as ablmnr'),
    ('vbn ab as abltere')
;
SELECT DISTINCT SUBSTR(`text`,POSITION("ab as" IN `text`),LENGTH(`text`)) 
FROM textlist WHERE `text` REGEXP 'ab as[[:blank:]]';


SUBSTR(`text`,POSITION("ab as" IN `text`),LENGTH(`text`))


ab as ablmnr

ab as abltere


SELECT DISTINCT SUBSTR(`text`,POSITION("ab as" IN `text`),LENGTH(`text`)) 
FROM textlist WHERE `text` REGEXP 'ab as[^[:blank:]]';


SUBSTR(`text`,POSITION("ab as" IN `text`),LENGTH(`text`))


ab astest

ab astestabc