def get_tokens(tokens):
    """Recursively gets tokens from a match list
    
    Args:
        tokens : List of tokens ['(', 'S', '(', 'NP', ')', ')']
    Returns:
        Stack of tokens
    """
    tokens = tokens[1:-1]
    ret = []
    start = 0
    stack = 0
    for i in range(len(tokens)):
        if tokens[i] == '(':
            if stack == 0:
                start = i
            stack += 1
        elif tokens[i] == ')':
            stack -= 1
            if stack < 0:
                raise Exception('Bracket mismatch: ' + str(tokens))
            if stack == 0:
                ret.append(get_tokens(tokens[start:i + 1]))
        else:
            if stack == 0:
                ret.append(tokens[i])
    if stack != 0:
        raise Exception('Bracket mismatch: ' + str(tokens))
    return ret