Multi language design serves several purposes, being inclusion and reachability the most important ones. In this small tutorial i'm going explain what i learned from Chatgpt about utf-8 validation that allows us to receive multibyte utf-8 characters and once verified they are valid, we consider the input valid. And than other validation methods can be employed to search for more intrinsic errors related to your business rules. From here on everything came from chatgpt i'm not a lier and i don't like to get credit from something i didn't do. So here we go. Let's go step by step and rigorously understand how UTF-8 byte sequences work, and why the hex constants (like 0xC0, 0xE0, etc.) are used in UTF-8 validation. 🧠 What is UTF-8? UTF-8 is a variable-length encoding for Unicode characters using 1 to 4 bytes per character. Each byte starts with a specific bit pattern that tells you what kind of byte it is: Byte Type Format (binary) Example (hex) Meaning 1-byte (ASCII) 0xxxxxxx 0x00–0x7F Standard ASCII (U+0000 to U+007F) 2-byte lead 110xxxxx 0xC0–0xDF Start of 2-byte sequence 3-byte lead 1110xxxx 0xE0–0xEF Start of 3-byte sequence 4-byte lead 11110xxx 0xF0–0xF7 Start of 4-byte sequence Continuation 10xxxxxx 0x80–0xBF Must follow a multi-byte lead ⚠️ Every byte after the lead must match 10xxxxxx, i.e., it must be in range 0x80–0xBF. 📦 How the Validator Works Let’s walk through what each part of the validator does: 🔹 Case 1: 1-byte (ASCII) if (bytes[i] <= 0x7F) If the byte is 0x00–0x7F, it's a valid ASCII char — move to next byte. 🔹 Case 2: 2-byte sequence else if ((bytes[i] & 0xE0) == 0xC0) ⚙️ Breakdown: bytes[i] & 0xE0 masks the first 3 bits: 11100000 We're testing for 110xxxxx — the lead byte of a 2-byte character 0xC0 = 11000000 So if bytes[i] & 0xE0 == 0xC0, we know it's a 2-byte lead ✅ Then check: (bytes[i + 1] & 0xC0) == 0x80 This ensures that the next byte is a continuation byte (10xxxxxx), which must be true for valid UTF-8. 🔹 Case 3: 3-byte sequence else if ((bytes[i] & 0xF0) == 0xE0) ⚙️ Breakdown: Mask top 4 bits: 11110000 0xE0 = 11100000 → matching 1110xxxx, the lead for a 3-byte sequence We check that the next 2 bytes are valid continuation bytes: (bytes[i+1] & 0xC0) == 0x80 && (bytes[i+2] & 0xC0) == 0x80 🔹 Case 4: 4-byte sequence else if ((bytes[i] & 0xF8) == 0xF0) ⚙️ Breakdown: Mask top 5 bits: 11111000 0xF0 = 11110000 → matches 11110xxx, the lead for a 4-byte UTF-8 char Check next 3 continuation bytes: (bytes[i+1] & 0xC0) == 0x80 && (bytes[i+2] & 0xC0) == 0x80 && (bytes[i+3] & 0xC0) == 0x80 ❌ Default: Invalid Byte else { return false; } If it doesn’t match any valid UTF-8 lead or continuation format → the sequence is invalid. 🧪 Example: How It Validates "日本" (U+65E5 U+672C) Encoded in UTF-8: E6 97 A5 E6 9C AC E6 → 11100110 = 3-byte lead 97 → 10010111 = continuation A5 → 10100101 = continuation ✅ Pass Then: E6 → 3-byte lead 9C → continuation AC → continuation ✅ Pass ✅ Final Takeaway The hex masks like 0xE0, 0xF0, 0xC0, etc., are used to extract the bit patterns that define UTF-8 byte types. The bitwise & tests identify which type of byte it is by focusing on the leading bits that encode structure. Let me know if you want a version of the validator that logs which byte failed or shows the character boundaries.