MySQL 8.0

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create table t(foo varchar(255));
insert into t (foo) values
('the quick brown fox'),
('jumped over the lazy dog');
select * from t;

foo

the quick brown fox

jumped over the lazy dog

select regexp_replace(foo, '[^a-z]+', '-') from t;

regexp_replace(foo, '[^a-z]+', '-')

the-quick-brown-fox

jumped-over-the-lazy-dog