why does the procedure hangs

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can anybody assist me with the following procedure?

it can compile and works when i do any of the following:

CALL ACTIVATE_ACCOUN T('')
CALL ACTIVATE_ACCOUN T('123')

but when i do a

CALL ACTIVATE_ACCOUN T('0394df...... ') <-- forty alnum chars long, the
server hangs as in infinite loop. (the pc doesn't hang)
it seems that any forty character long input produces the error.

CREATE PROCEDURE ACTIVATE_ACCOUN T(IN p_uid CHAR(40))
BEGIN
IF(p_uid REGEXP '^[a-zA-Z0-9]{40}$') THEN
BEGIN
UPDATE
account,
member
SET
activated = 1
WHERE
account.id = member.account_ id
AND
SHA1(CONCAT(acc ount_id, email_address)) = p_uid AND activated = 0;
END;
END IF;
END