UPPER, LOWER, SUBSTRING, REPLACE

SELECT UPPER('hello');     -- HELLO
SELECT LOWER('Hello');     -- hello
SELECT INITCAP('hello world');  -- 'Hello World' (PostgreSQL extension)

SELECT * FROM users WHERE LOWER(email) = LOWER('USER@Example.COM');

-- Or use a functional index:
CREATE INDEX users_email_lower_idx ON users (LOWER(email));

CREATE EXTENSION IF NOT EXISTS citext;
CREATE TABLE users (email CITEXT UNIQUE);

SELECT SUBSTRING('PostgreSQL', 1, 4);    -- 'Post'
SELECT SUBSTRING('PostgreSQL' FROM 5);    -- 'greSQL'

-- 1-based indexing, not 0-based

SELECT SUBSTRING('user@example.com' FROM '@(.+)$');
-- 'example.com'

SELECT LEFT('PostgreSQL', 4);     -- 'Post'
SELECT RIGHT('PostgreSQL', 3);    -- 'SQL'

SELECT REPLACE('hello world', 'world', 'SQL');   -- 'hello SQL'
SELECT REPLACE(phone, '-', '');                  -- strip dashes

SELECT REGEXP_REPLACE('Order #1234', '[^0-9]', '', 'g');  -- '1234'
SELECT REGEXP_REPLACE(text, '\s+', ' ', 'g');             -- collapse whitespace

SELECT LENGTH('héllo');           -- 5
SELECT CHAR_LENGTH('héllo');      -- 5
SELECT OCTET_LENGTH('héllo');     -- 6  (UTF-8 byte length)

SELECT POSITION('SQL' IN 'PostgreSQL');    -- 8
SELECT STRPOS('PostgreSQL', 'SQL');        -- 8

SELECT REVERSE('hello');     -- 'olleh'