MySQL 8.0

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CREATE TABLE test
SELECT '1' data UNION
SELECT '123' UNION
SELECT '123 456' UNION
SELECT '1asdf' UNION
SELECT '111111111111111111111111111111111111111111111' UNION
SELECT ' 123' UNION
SELECT 'a123' UNION
SELECT 'asdf';

Records: 8  Duplicates: 0  Warnings: 0


SELECT data, data REGEXP '^\\d'
FROM test;


data
data REGEXP '^\d'


1
1

123
1

123 456
1

1asdf
1

111111111111111111111111111111111111111111111
1

 123
0

a123
0

asdf
0


SELECT data
FROM test
WHERE data REGEXP '^\\d';


data


1

123

123 456

1asdf

111111111111111111111111111111111111111111111

data	data REGEXP '^\d'
1	1
123	1
123 456	1
1asdf	1
111111111111111111111111111111111111111111111	1
123	0
a123	0
asdf	0