Single header utf8 string functions for C and C++. [Unlicense]

Unicode routines (UTF8, UTF16, UTF32): billions of characters per second using SSE2, AVX2, NEON, AVX-512. [Apache-2/MIT]

A clean C library for processing UTF-8 Unicode data. [MIT]

Unicode Algorithms Implementation for C/C++. [Unlicense or MIT]

A set of C/C++ and Java libraries providing Unicode and Globalization support. [ICU]

An encoding conversion library between different character encodings. [GPL]