import re


def detect_repeat(text, expression=r"([\u4e00-\u9fa5])(\1+)"):
    """detect repeat characters

    Args:
        text (str): text
        expression (regexp, optional): regular expression . Defaults to r"([\u4e00-\u9fa5])(\1+)".

    Returns:
        (list): repeat characters

    Examples:
        >>> s = String()
        >>> text = "锄禾日当当午，汗滴禾下下土"
        >>> s.detect_repeat(text)
        ['当当', '下下']
    """
    pattern = re.compile(expression)
    matchs = re.finditer(pattern, text)
    results = []
    for match in matchs:
        results.append(match.group())
    return results