I use the siunitx-package to typeset numbers and units with correct spacing etc. There is a command


for that. However, it would be much easier to write something like

\xSI{<number and unit>}

and let LaTeX automatically split the number and the unit and call \SI{}{} internally.

\xSI{9.81m/s^2}  →  \SI{9.81}{m/s^2}
\xSI{1e-3m}      →  \SI{1e-3}{m}

I know that expl3 provides some regex-functionality, but as I am only familiar with "regular" LaTeX I do not know where to start.


How can I write a wrapper function for SI{}{} (using expl3?) so that the number and the unit parts get automatically split?

  • 1
    So all the trouble to save }{ keystrokes ?
    – percusse
    Apr 7 '13 at 21:17
  • I was thinking the same as percusse. Is this really your final intention? Apr 7 '13 at 21:20
  • @percusse Why not use a proper editor with Auto-Completion? \SI followed by Ctrl + <Space> gives me \SI{<cursor>}{} where I can easily “over-tab” the }{. Apr 7 '13 at 21:21
  • No, I'm not that lazy – I had to convert a pdf back to LaTeX, with lots of units in it. I wrote a script that fixes umlauts, unicode characters and line breaks, so I could copy-paste most of the document. However, it was a pain to fix the units, because pasting gave me something like 9, 81 m/s2 (not the spaces). I thought of a command in which I could wrap everything unit-related and LaTeX would do the rest. Also, this could be a good way to show what is possible in LaTeX3 and a good starting point for me to get more familiar with the expl3-syntax. Apr 10 '13 at 21:16

You find a regular expression for numbers in scientific format here

\NewDocumentCommand{\xSI}{ O{} m }
  \ralfix_split:n { #2 }
  \ralfix_si:n { #1 }

\seq_new:N \l__ralfix_pieces_seq
\cs_new_protected:Npn \ralfix_split:n #1
    ( [ - + ]? [0-9]* [\.\,]? [0-9]+ ( [eE] [-+]? [0-9]+ )? )
   { #1 }
\cs_new_protected:Npn \ralfix_si:n #1
    \exp_not:n {\SI[#1]}
    {\seq_item:Nn \l__ralfix_pieces_seq {2}}
    {\seq_item:Nn \l__ralfix_pieces_seq {4}}



The function could be refined by first examining item number 1 in the resulting sequence, that will be non empty if no match has been found.

This said, I would recommend you using \SI{...}{...} which is not much more difficult to type and clearly separates different things from each other.

enter image description here

  • I had to add unit-formatting afterwards, so autocomplete was not a big help. Your solution is just perfect, thank you very much! Apr 10 '13 at 21:20

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