A sequence of 7-bit bytes is both valid ASCII and valid UTF-8, and under either interpretation represents the same sequence of characters. Therefore, the 7-bit bytes in a UTF-8 stream represent all and only the ASCII characters in the stream. Thus, many text processors, parsers, protocols, file formats, text display programs, etc., which use ASCII characters for formatting and control purposes, will continue to work as intended by treating the UTF-8 byte stream as a sequence of single-byte characters, without decoding the multi-byte sequences. ASCII characters on which the processing turns, such as punctuation, whitespace, and control characters will never be encoded as multi-byte sequences. It is therefore safe for such processors to simply ignore or pass-through the multi-byte sequences, without decoding them. For example, ASCII whitespace may be used to tokenize a UTF-8 stream into words; ASCII line-feeds may be used to split a UTF-8 stream into lines; and ASCII NUL characters can be used to split UTF-8-encoded data into null-terminated strings. Similarly, many format strings used by library functions like "printf" will correctly handle UTF-8-encoded input arguments. Fallback and auto-detection: Only a small subset of possible byte strings are a valid UTF-8 string: several bytes cannot appear; a byte with the high bit set cannot be alone; and further requirements mean that it is extremely unlikely that a readable text in any extended ASCII is valid UTF-8. Part of the popularity of UTF-8 is due to it providing a form of backward compatibility for these as well. A UTF-8 processor which erroneously receives extended ASCII as input can thus "auto-detect" this with very high reliability. A UTF-8 stream may simply contain errors, resulting in the auto-detection scheme producing false positives; but auto-detection is successful in the vast majority of cases, especially with longer texts, and is widely used. It also works to "fall back" or replace 8-bit bytes using the appropriate code-point for a legacy encoding when errors in the UTF-8 are detected, allowing recovery even if UTF-8 and legacy encoding is concatenated in the same file. Prefix code: The first byte indicates the number of bytes in the sequence. Reading from a stream can instantaneously decode each individual fully received sequence, without first having to wait for either the first byte of a next sequence or an end-of-stream indication. The length of multi-byte sequences is easily determined by humans as it is simply the number of high-order