[texhax] Low-level TeX question: string substitution macro

[CV Radhakrishnan]

On Tue, May 29, 2007 7:08 pm, Toby Cubitt said:
>
> I'm trying to write an internal macro that does string substitution, in
> order to escape certain characters in the string before writing it to a
> file. (The package is supposed to be writing a sed script, so I need to
> escape characters that have a special meaning in regular expressions.)
>
> If this was a user-level macro to be used in the LaTeX source itself, I
> think can see how it could be done, by changing the catcodes of the
> characters to be escaped to 13 (active character), then defining these
> active characters to expand to escaped versions of themselves. (I
> suppose this would be somewhat akin to LaTeX's \verb command). The
> trouble is, this macro is to be used in a LaTeX package, and I need
> something like the following to work:
>
>
> \begingroup%
> \catcode`|=0
> |catcode`.=13 |catcode`[=13 |catcode`]=13
> |catcode`^=13 |catcode`$=13 %$
> \catcode`\\=13
> |gdef|@escapechars#1{%
>    |begingroup
>    |catcode`|=0
>    |catcode`.=13 |catcode`[=13 |catcode`]=13
>    |catcode`^=13 |catcode`$=13 %$
>    |catcode`\=13
>    |def\{|string\|string\}%
>    |def^{|string\|string^}%
>    |def${|string\|string$}%
>    |def.{|string\|string.}%
>    |def[{|string\|string[}%
>    |def]{|string\|string]}%
>    #1|endgroup%
> }
> |endgroup%
> \def\@tmpa{\foobar}
> \expandafter\@escapechars\expandafter{\@tmpa}%

[...]

This will work with a slight modification as given below:

|gdef|@escapechars{%
%   |begingroup
   |catcode`|=0
   |catcode`.=13 |catcode`[=13 |catcode`]=13
   |catcode`^=13 |catcode`$=13 %$
   |catcode`\=13
   |def\{|string\|string\}%
   |def^{|string\|string^}%
   |def${|string\|string$}%
   |def.{|string\|string.}%
   |def[{|string\|string[}%
   |def]{|string\|string]}%
%   #1|endgroup%
}
|endgroup%

{\tt\@escapechars \foobar ^FOO $foo}

-- 
Radhakrishnan