scientific notationsiunitx
In chemistry, one often has to use decimal exponents: 10^{-4.6} for the value of a reaction constant. I'm usually using siunitx to type numbers in scientific notation with \num{4e5} for instance. However recent versions don't allow decimal exponents anymore. I'm now left with many files filled with commands like \num{1e-5.6} that don't compile anymore.
What other package would parse numbers with decimal exponents ?
Here I introduce a secondary parser \parseuncertainty which will separate out the mean value from the variation, and stuff them into two separate tokens. Then, your \SI command can compare against \SImeanvalue rather than #2.
\documentclass{article}
\RequirePackage[
per-mode=reciprocal,
scientific-notation=true,
retain-explicit-plus,
table-space-text-post=\textsuperscript{~a},
table-align-text-post=true,
table-align-exponent,
table-align-uncertainty,
separate-uncertainty = true,
]{siunitx}
\usepackage{expl3}
\ExplSyntaxOn
\cs_new_eq:NN \fpcmpTF \fp_compare:nTF
\ExplSyntaxOff
\let\OldSI\SI%
\renewcommand*{\SI}[3][]{%
\parseuncertainty#2()\relax% NEW
\fpcmpTF{abs(\SImeanvalue)<=.001}{% ALTERED
\OldSI[scientific-notation=true,#1]{#2}{#3}%
}{%
\fpcmpTF{abs(\SImeanvalue)>=1000}{% ALTERED
\OldSI[scientific-notation=true,#1]{#2}{#3}%
}{%
\OldSI[scientific-notation=false,#1]{#2}{#3}%
}%
}%
}
\def\parseuncertainty#1(#2)#3\relax{% NEW
\def\SImeanvalue{#1}\def\SIuncertainty{#2}}% NEW
\begin{document}
\SI{83}{\mega\pascal} looks good, \SI{83(22)}{\mega\pascal} does too.\par
And \SI{12345(456)}{\mega\pascal} also.
\end{document}
As Joseph Wright explains in his comment, siunitx does not attempt to handle this, because human judgement is needed in deciding what the numbers mean and how to round them.
If you want to automate this, you will have to use a different tool.
Best Answer
You could write a macro yourself, assuming that you don't have any complicated use cases. Of course, you won't have access to any of the other nice features of
siunitx.