\char can be used to write invalid raw characters

Author: anderskCreated Sep 25, 2020Updated Aug 2, 2026

Labelsbug

Using \char, KaTeX can be convinced to output raw characters that are invalid HTML5 and/or XML:

\char0: NUL character (invalid HTML5, XML)
\char1–\char8, \char11, \char14–\char31: C0 control codes (invalid HTML5)
\char12: form feed (invalid XML)
\char127: DEL character (invalid HTML5)
\char128–\char159: C1 control codes (invalid HTML5)
\char55296–\char57343: unpaired surrogates (invalid HTML5, XML)
\char64976–\char65007: noncharacters (invalid HTML5)
\char65534, \char65535: noncharacters (invalid HTML5, XML)

The number is interpreted modulo 65536, so \char65536 also outputs a NUL character, etc.

> katex = require('katex'); katex.version
'0.12.0'
> katex.renderToString('\\char0').includes('\x00')  // NUL character
true
> katex.renderToString('\\char1').includes('\x01')  // C0 control code
true
> katex.renderToString('\\char12').includes('\f')  // form feed
true
> katex.renderToString('\\char127').includes('\x7f')  // DEL character
true
> katex.renderToString('\\char128').includes('\x80')  // C1 control code
true
> katex.renderToString('\\char55296').includes('\ud800')  // unpaired surrogate
true
> katex.renderToString('\\char64976').includes('\ufdd0')  // noncharacter
true
> katex.renderToString('\\char65534').includes('\ufffe')  // noncharacter
true
> katex.renderToString('\\char65536').includes('\x00')  // NUL character again
true

Source: KaTeX/KaTeX

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