import re

def lreplace(pattern, sub, string):
    """
    Replaces 'pattern' in 'string' with 'sub' if 'pattern' starts 'string'.
    """
    return re.sub('^%s' % pattern, sub, string)

def rreplace(pattern, sub, string):
    """
    Replaces 'pattern' in 'string' with 'sub' if 'pattern' ends 'string'.
    """
    return re.sub('%s$' % pattern, sub, string)