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Decoding: UTF-8 Leading Byte Sequence Length Tool

Classify one byte as a valid RFC3629 UTF-8 lead byte and its nominal sequence length; continuation/invalid leads return length zero, not full-sequence validity.

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Description

Inspect a UTF-8 lead octet's expected sequence length.

Unicode code points, UTF-16 units and UTF-8 octets are different representations. A local escape or byte check can report useful structure without establishing that an entire encoded string is well formed.

Inspect a UTF-8 lead octet's expected sequence length.

When to use this tool

Use Decoding: UTF-8 Leading Byte Sequence Length when inspecting the stated scalar, escape or byte-layout component.

How to use the tool and how it works

  1. Enter byte. Use the units and conventions below.
  2. Run the tool. Correct any format, range, or cross-field input error before interpreting the output.
  3. Read result. Expand result to inspect the returned fields.

Inspect a UTF-8 lead octet's expected sequence length. Require whole byte in [0,255]; return lengths 1 for ASCII, 2 for C2-DF, 3 for E0-EF, 4 for F0-F4, otherwise zero and invalid-leading flag.1

Unicode scalar values exclude surrogate code points. Code units, scalar values and encoded byte sequences are distinct representations.2

Input fieldMeaning and unitsAccepted input
byteOne numeric octet, from 0 through 255.Required. Safe integer.
Output fieldInterpretationReturned shape
resultReturned object; expand its named members below.Required. Object.
result.sequenceLengthSequence LengthCalculated result member: number.
result.validLeadingByteWhether the octet can begin a modern UTF-8 sequence; continuation bytes and C0/C1/F5..FF are not valid starts.Calculated result member: boolean.
json
{
  "input": {
    "byte": 240
  },
  "output": {
    "result": {
      "sequenceLength": 4,
      "validLeadingByte": true
    }
  }
}

Limitations and assumptions

  • A permitted lead byte does not validate following bytes, scalar range or a complete UTF-8 sequence.
  • Outputs are nested inside result. Inputs must satisfy both the field types and the cross-field conditions above. Invalid input causes an error; a successful false flag or documented placeholder is a different result.
  • Numeric calculations use binary floating point unless exact decimal-integer arithmetic is explicitly stated. No hidden measurement tolerance or decimal rounding is added; nonfinite or out-of-contract results are rejected. Numeric signed-zero results are exported as zero.

Alternative or Complementary methods

Use a conforming complete decoder for strings and distinguish Unicode scalar validity from source-language or HTML error recovery.

References

  1. Calculation and input contract: decode/utf8-leading-byte-sequence-length — tool.io

  2. Unicode 17.0: conformance and encoding forms — Unicode Consortium

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