import re


def get_build_number_from_body(body):
    """Extracts the build number from the given body.
    
    Arguments:
        body {dict} -- The body as extracted by `extract_body()`
    
    Returns:
        str -- The build number if given. If there's an error extracting this
               or if the build number is malformed, None is returned.
    """
    if 'build_number' in body:
        build_number = body['build_number']
        if re.match("^[1-9][0-9]*$", build_number):
            return build_number
    return None