In my preamble I have:


Suppose in some math context I use:


while in some other I have:


My problem

I have noticed that in the future I may want to change all \myvecb{F}_y to \myvecb{F_y} (or, in some other context, perhaps vice versa). Instead of doing everything manually (because a simple regex replace may too complicated for some reason), is it possible to define \myvecb command definition such that I can manipulate the stated-problem within the definition?

  • 2
    Yes. \makeatletter \newcommand*\myvecb[1]{\@ifnextchar_{\myvecbaux{#1}}{\vb{#1}}} \def\myvecbaux#1_#2{\vb{#1_#2}} \makeatother.
    – Manuel
    Jul 6, 2017 at 10:53
  • @Manuel Thank you. Is your solution just for one character? (For instance I may have \myvecb{F}_{A|B}.
    – blackened
    Jul 6, 2017 at 11:20
  • 1
    Ah, add braces around #2 in \vb{#1_{#2}}.
    – Manuel
    Jul 6, 2017 at 11:33
  • Super easy with xparse, I think the new e type was made for this.
    – daleif
    Jul 6, 2017 at 19:23

1 Answer 1


This checks if a _ is following and in that case it grabs the following _{..} and puts it as a subscript inside \vb{..}.


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