Using Unicode (UTF-8) in C++

Currently, I have to deal with Unicode in C++ 11 (Linux environment). UTF-8 is used as default encoding. Tasks that I need:

• Replace.
• Regex
• Iterate through a UTF-8 string. I don't know if using std::string and "for (character c : s)" will do what I want 'cause each character must be a unicode character. For example ế is one character, mão is a word contains 3 characters
• Substring.
• Concatenate substring with unicode characters or concatenate unicode characters.
• Length.
• Trim.
• Read and write files.

What library should I use to achieve the best result?

Thank you very much. Looking forward to hearing from you soon.