PostgreSQL function to determine network prefix from a phone number

network_operator_function.sql

CREATE OR REPLACE FUNCTION network_operator(msisdn text)
RETURNS text AS 
$$
DECLARE
    operator TEXT;
BEGIN
    IF msisdn ~ '^(0|234)(703|706|803|806|810|813|816)\\d+$' THEN
        operator := 'MTN';
    ELSEIF msisdn ~ '^(0|234)(705|805|807|815)\\d+$' THEN
        operator := 'GLO';
    ELSEIF msisdn ~ '^(0|234)(708|802|808|812)\\d+$' THEN
        operator := 'AIRTEL';
    ELSEIF msisdn ~ '^(0|234)(809|817|818)\\d+$' THEN
        operator := 'ETISALAT';
    ELSE
        operator := 'UNKNOWN';
    END IF;
    
    RETURN operator;
END
$$
LANGUAGE plpgsql;