Unescape a value.

def _unescape_value(value):
    """Unescape a value."""
    def unescape(c):
        return {
            "\\\\": "\\",
            "\\\"": "\"",
            "\\n": "\n",
            "\\t": "\t",
            "\\b": "\b",
            }[c.group(0)]
    return re.sub(r"(\\.)", unescape, value)