Checking for a macro in a string without expanding it

[Joey McCollum via ntg-context <ntg-context@ntg.nl>]

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As the subject of this question suggests, this is really more of a question
about expansion control (a topic that is still a bit obscure to me).
Suppose I have a macro \inner that expects a single argument or an
assignment of parameters in brackets. For my purposes, I don't want this
macro to do anything when it is typeset, so I'll just define it as empty:

```
\def\inner[#1]\empty
```

Now suppose I have another macro \outer that invokes this macro with some
specific input and sets some plain text after it:

```
\def\outer{\inner[123] etc.}
```

What I'd like to do is parse the argument of \inner in \outer. I was hoping
that a string search in Lua would work, but I'm not having any luck. A
minimal (non)-working example is included below:

```

\def\inner[#1]\empty

\def\outer{\inner[123] etc.}


\startluacode

local userdata = userdata or {}

function userdata.parseinner(str)

local innerparams = ""

    if string.find(str, "\\inner(%b[])") then

      i, j = string.find(str, "\\inner(%b[])")

      innerparams = string.sub(str, i+1, j-1) -- we just want the content
inside the brackets

    end

    context(innerparams)

    return

end

\stopluacode

\def\parseinner#1{\ctxlua{userdata.parseinner([==[#1]==])}}


\starttext

Testing:\blank

\parseinner{\outer}

\stoptext
```

My problem is that when I pass \outer to the \parseinner macro, it gets
fully expanded, so there isn't anything left to match "\\inner%b[]". Is
there a way to expand \outer when I pass it to the \parseinner macro
without also expanding the \inner macro inside it? Or is there some other
preferred way of doing this?

Joey

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