Postgres and alias

Hi, I am migrating a database from Sybase to PostgreSql 12.

This select in sybase works for me, but with postgresql it accuses me
"THERE IS NO COLUMN ls_numero"

select '1234567890' as ls_number,
                   substr (ls_number, 3, 3);

Is it possible to get information from an alias in postgresql? how would the
code be?