<p>Yes its the lexer, <a href="https://github.com/geany/geany/blob/master/scintilla/lexers/LexLaTeX.cxx#L268">https://github.com/geany/geany/blob/master/scintilla/lexers/LexLaTeX.cxx#L268</a> which says any [ after (optional) whitespace is the start of options.</p>

<p>I assume latex itself gets around it because it knows the <code>\in</code> is a no-argument command, but the lexer won't know that, its purely syntax only, so I'm not sure how it can do anything different to what it is doing.</p>

<p>You could try reporting it to Scintilla anyway. </p>

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