Conditional based on character following the macro

Question

Basically, I would like to something like \xspace does, i.e. the meaning of the macro should depend on what is following it. So for example if I say

\mymacro a

it might expand to foo, and if any other character is following, it should expand to bar. In my specific case I'll try to match a unicode character, if that makes a difference.

Edit: Specifically, I want to influence the spacing after an integral. When it stands alone, the default spacing is fine. If it is the first of multiple integrals, the space became too wide. Here is what I cobbled together from the previous answer:

\newcommand\timeintegral[1]{
    ∫_{t_0}^{t_1} \if ∫#1 \!\!\! \fi #1
}

This is how it looks in use:

\timeintegral E \, dt = \timeintegral ∫_z ∫_φ f \, dφ dz dt

Answer 1

This is a straightforward application of \@ifnextchar:

\documentclass{article}
\usepackage{unicode-math}
\setmainfont{TeX Gyre Pagella}
\setmathfont{Asana Math}

\makeatletter
\newcommand{\timeintegral}{%
   ∫_{t_0}^{t_1} \@ifnextchar∫{\!\!\!}{}%
}
\makeatother

\begin{document}
\[
\timeintegral E \, dt = \timeintegral ∫_z ∫_φ f \,dφ\,dz\,dt
\]
\end{document}

The LaTeX3 version:

\documentclass{article}
\usepackage{unicode-math}
\setmainfont{TeX Gyre Pagella}
\setmathfont{Asana Math}

\ExplSyntaxOn
\NewDocumentCommand{\timeintegral}{}
 {
  ∫\sb{t\sb{0}}^{t\sb{1}} \psirus_check_int:
 }
\cs_new_protected:Npn \psirus_check_int:
 {
  \peek_charcode_ignore_spaces:NT ∫ { \!\!\! }
 }
\ExplSyntaxOff

\begin{document}
\[
\timeintegral E \, dt = \timeintegral ∫_z ∫_φ f \,dφ\,dz\,dt
\]
\end{document}