R:使用for循环查找另一个字符串旁边的特定字符串

def find_adjacent_string(target_string, specific_string):
    specific_string_length = len(specific_string)
    target_string_length = len(target_string)

    for i in range(target_string_length):
        if target_string[i] == specific_string[0]:
            if target_string[i:i+specific_string_length] == specific_string:
                return target_string[i-specific_string_length:i] + specific_string + target_string[i+specific_string_length:i+2*specific_string_length]

    return "未找到特定字符串旁边的字符串"

# 示例用法
target_string = "Hello World! This is a test string."
specific_string = "is"

result = find_adjacent_string(target_string, specific_string)
print(result)