def _resolve_datacenter(dc, pillarenv):
    '''
    If ``dc`` is a string - return it as is.
    If it's a dict then sort it in descending order by key length and try
    to use keys as RegEx patterns to match against ``pillarenv``.
    The value for matched pattern should be a string (that can use
    ``str.format`` syntax togetehr with captured variables from pattern)
    pointing to targe data center to use.
    If none patterns matched return ``None`` which meanse us datacenter of
    conencted Consul agent.
    '''
    log.debug('Resolving Consul datacenter based on: %s', dc)
    try:
        mappings = dc.items()
    except AttributeError:
        log.debug('Using pre-defined DC: \'%s\'', dc)
        return dc
    log.debug('Selecting DC based on pillarenv using %d pattern(s)', len(mappings))
    log.debug('Pillarenv set to \'%s\'', pillarenv)
    sorted_mappings = sorted(mappings, key=lambda m: (-len(m[0]), m[0]))
    for pattern, target in sorted_mappings:
        match = re.match(pattern, pillarenv)
        if match:
            log.debug('Matched pattern: \'%s\'', pattern)
            result = target.format(**match.groupdict())
            log.debug('Resolved datacenter: \'%s\'', result)
            return result
    log.debug(
        'None of following patterns matched pillarenv=%s: %s',
        pillarenv, ', '.join(repr(x) for x in mappings)
    )