Thanks! Invoking string.find(str,"^"..pre) is certainly much easier. This
sort of thing would be a nice general-purpose helper function. Denis Maier
and I should have a "wish list" of desired helpers and features (mostly
related to bibliographies/publication support) ready for you (and Alan)
sometime soon.

But for now, it looks like I still need to work out errors that arise when
I pass a macro as the "str" input to be searched. I've defined a \loc macro
that accepts an assignment and outputs a formatted string, and later, I
want to check if this macro (after it has been fully expanded) starts with
a certain prefix. A minimal (non-)working example follows:

```

\starttexdefinition loc [#1]

\doifassignmentelse{#1} {

% if an assignment, then parse and format accordingly

\getparameters[loc][#1]

% Was a section number specified?

\doifdefined{locsec} {

§ \locsec\btxcomma

}

} {

% otherwise, just print the input as-is

#1

}

\stoptexdefinition


\def\DoIfPrefixElse#1#2{\ctxlua{commands.doifelse(string.find("#2","^".."#1"))}}


\def\currentbtxloctext{\loc[sec=31]}


\starttext

\DoIfPrefixElse{§}{\currentbtxloctext}{YES}{NOP}

\stoptext
```

Lua is throwing an "invalid escape sequence near '"\l'" error, which is
presumably because the macro \loc[sec=31] is not being expanded. How do I
fix this?

Thanks!

Joey

On Fri, Nov 26, 2021 at 3:46 AM Hans Hagen via ntg-context <
ntg-context@ntg.nl> wrote:

> On 11/26/2021 7:42 AM, Joey McCollum via ntg-context wrote:
> > I wasn't aware of a general-purpose "doifstartswith" macro in ConTeXt
> > (the \doifnextcharelse macro only works one character at a time, and the
> > \doifinstring macros may capture substrings that are not prefixes), and
> > I'd like to develop one for something I'm working on. I've been trying
> > to do this in Lua, as that seemed like the most natural approach.
> > Normally, something like this would work fine as a foundation:
> >
> > ```
> >    function isprefix(prefix, str)
> >      if string.sub(str, 1, string.len(prefix)) == prefix then
> >        return true
> >      end
> >      return false
> >    end
> > ```
>
> how about
>
> if string.find(str,"^"..prefix) then
>
> in:
>
> \starttext
>
>
> \def\DoIfPrefixElse#1#2{\ctxlua{commands.doifelse(string.find("#2","^".."#1"))}}
>
> \DoIfPrefixElse{pre}{prefix}{YES}{NOP}
> \DoIfPrefixElse{pre}{suffix}{YES}{NOP}
>
> \stoptext
>
> utf strings are just sequences of bytes so matching works
>
> when you want to do more in lua you can decide for
>
> \startluacode
> interfaces.implement {
>      name      = "DoIfPrefixElse",
>      arguments = { "argument", "argument" },
>      actions   = function(pre,str)
>          commands.doifelse(string.find(str,"^"..pre))
>      end
> }
> \stopluacode
>
> \DoIfPrefixElse{pre}{prefix}{YES}{NOP}
> \DoIfPrefixElse{pre}{suffix}{YES}{NOP}
>
> but in any case: make sure that you don't clash with built in ...
>
> if needed i can make a set of fast(er) ones but someone has to collect a
> list of 'handy helpers' first
>
> Hans
>
>
> -----------------------------------------------------------------
>                                            Hans Hagen | PRAGMA ADE
>                Ridderstraat 27 | 8061 GH Hasselt | The Netherlands
>         tel: 038 477 53 69 | www.pragma-ade.nl | www.pragma-pod.nl
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