Programming language: Rust
License: GNU General Public License v3.0 or later
Tags: Encoding     Binary     Text     Yaml     Unicode     Invalid     Repr    
Latest version: v0.2.4

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STFU-8: Sorta Text Format in UTF-8

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STFU-8 is a hacky text encoding/decoding protocol for data that might be not quite UTF-8 but is still mostly UTF-8. It is based on the syntax of the repr created when you write (or print) binary text in rust, python, C or other common programming languages.

Its primary purpose is to be able to allow a human to visualize and edit "data" that is mostly (or fully) visible UTF-8 text. It encodes all non visible or non UTF-8 compliant bytes as longform text (i.e. ESC becomes the full string r"\x1B"). It can also encode/decode ill-formed UTF-16.

Comparision to other formats:

  • UTF-8 (i.e. std::str): UTF-8 is a standardized format for encoding human understandable text in any language on the planet. It is the reason the internet can be understood by almost anyone and should be the primary way that text is encoded. However, not everything that is "UTF-8 like" follows the standard exactly. For instance:
    • The linux command line defines ANSI escape codes to provide styles like color, bold, italic, etc. Even though almost everything printed to a terminal is UTF-8 text these "escape codes" might not be, and even if they are they are UTF-8, they are not visible characters.
    • Windows paths are not necessarily UTF-8 compliant as they can have [ill formed text][utf-16-ill-formed-text].
    • There might be other cases you can think of or want to create. In general, try not to create more use cases if you don't have to.
  • rust's OsStr: OsStr is the "cross platform" type for handling system specific strings, mainly in file paths. Unlike STFU-8 it not (always) coercible into UTF-8 and therefore cannot be serialized into JSON or other formats.
  • WTF-8 (rust-wtf8): is great for interoperating with different UTF standards but cannot be used to transmit data over the internet. The spec states: "WTF-8 must not be used to represent text in a file format or for transmission over the Internet."
  • base64 (base64): also encodes binary data as UTF-8. If your data is actually binary (i.e. not text) then use base64. However, if your data was formerly text (or mostly text) then encoding to base64 will make it completely un(human)readable.
  • Array[u8]: obviously great if your data is actually binary (i.e. NOT TEXT) and you don't need to put it into a UTF-8 encoding. However, an array of bytes (i.e. ['0x72', '0x65', '0x61', '0x64', '0x20', '0x69', '0x74'] is not human readable. Even if it were in pure ASCII the only ones who can read it efficiently are low-level programming Gods who have never figured out how to debug-print their ASCII.
  • STFU-8 (this crate): is "good" when you want to have only printable/hand-editable text (and your data is mostly UTF-8) but the data might have a couple of binary/non-printable/ill-formed pieces. It is very poor if your data is actually binary, requiring (on average) a mapping of 4/1 for binary data.


In simple terms, encoded STFU-8 is itself always valid unicode which decodes to binary (the binary is not necessarily UTF-8). It differs from unicode in that single \ items are illegal. The following patterns are legal:

  • \\: decodes to the backward-slash (\) byte (\x5c)
  • \t: decodes to the tab byte (\x09)
  • \n: decodes to the newline byte (\x0A)
  • \r: decodes to the linefeed byte (\x0D)
  • \xXX where XX are exactly two case-insensitive hexidecimal digits: decodes to the \xXX byte, where XX is a hexidecimal number (example: \x9F, \xaB or \x05). This never gets resolved into a code point, the value is pushed directly into the decoder stream.
  • \uXXXXXX where XXXXXX are exacty six case-insensitive hexidecimal digits, decodes to a 24bit number that typically represenents a unicode code point. If the value is a unicode code point it will always be decoded as such. Otherwise stfu8 will attempt to store the value into the decoder (if the value is too large for the decoding type it will be an error).

stfu8 provides 2 different categories of functions for encoding/decoding data that are not necessarily interoperable (don't decode output created from encode_u8 with decode_u16).

  • encode_u8(&[u8]) -> String and decode_u8(&str) -> Vec<u8>: encodes or decodes an array of u8 values to/from STFU-8, primarily used for interfacing with binary/nonvisible data that is almost UTF-8.
  • encode_u16(&[u16]) -> String and decode_u16(&str) -> Vec<u16>: encodes or decodes an array of u16 values to/from STFU-8, primarily used for interfacing with legacy UTF-16 formats that may contain [ill formed text][utf-16-ill-formed-text] but also converts unprintable characters.

There are some general rules for encoding and decoding:

  • If \u... cannot be resolved into a valid UTF code point it must fit into the decoder. For instance, trying to decode "\x00DEED" (which is an UTF-16 Trail surrogage) using decode_u8 will fail, but will succeed with decode_u16.
  • No escaped values are ever chained. For example, "\x01\x02" will be [0x01, 0x02] not [0x0102] -- even if you use decode_u16.
  • Values escaped with \x... are always copied verbatum into the decoder. I.e. \xFF is a valid UTF-32 code point, but if decoded with decode_u8 it will be 0xFE in the buffer, not two bytes of data as the UTF-8 character 'þ'. Note that with decode_u16 0xFE is a valid UTF-16 code point, so when re-encoded would be the 'þ' character. Moral of the story: don't mix inputs/outputs of the the u8 and u16 functions.

tab, newline, and line-feed characters are "visible", so encoding with them in "pretty form" is optional.

UTF-16 Ill Formed Text

The problem is succinctly stated here:


Q: How do I convert an unpaired UTF-16 surrogate to UTF-8?

A different issue arises if an unpairedsurrogate is encountered when converting ill-formed UTF-16 data. By represented such an unpaired surrogate on its own as a 3-byte sequence, the resulting UTF-8 data stream would become ill-formed. While it faithfully reflects the nature of the input, Unicode conformance requires that encoding form conversion always results in valid data stream. Therefore a convertermust treat this as an error. [AF]

Also, from the WTF-8 spec

As a result, [unpaired] surrogates do occur in practice and need to be preserved. For example:

In ECMAScript (a.k.a. JavaScript), a String value is defined as a sequence of 16-bit integers that usually represents UTF-16 text but may or may not be well-formed. Windows applications normally use UTF-16, but the file system treats path and file names as an opaque sequence of WCHARs (16-bit code units).

We say that strings in these systems are encoded in potentially ill-formed UTF-16 or WTF-16.

Basically: you can't (always) convert from UTF-16 to UTF-8 and it's a real bummer. WTF-8, while kindof an answer to this problem, doesn't allow me to serialize UTF-16 into a UTF-8 format, send it to my webapp, edit it (as a human), and send it back. That is what STFU-8 is for.


The source code in this repository is Licensed under either of

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

The STFU-8 protocol/specification itself (including the name) is licensed under CC0 Community commons and anyone should be able to reimplement or change it for any purpose without need of attribution. However, using the same name for a completely different protocol would probably confuse people so please don't do it.

*Note that all licence references and agreements mentioned in the stfu8 README section above are relevant to that project's source code only.