Macro char `#` doubles when made letter?
\documentclass{article}
\makeatletter
\def\allowhash{%%%%
\bgroup\everyeof{\egroup}\catcode35=11\relax\scantokens}%
\makeatother
\begin{document}
\allowhash{One hash: #, not two}
\end{document}
or in a macro as requested in comments
\documentclass{article}
\makeatletter
\def\allowhash{%%%%
\bgroup\catcode35=11\relax\afterassignment\egroup\gdef\foo}%
\makeatother
\begin{document}
\allowhash{One hash: #, not two}
[\foo] [\foo]
\end{document}
When \scantokens acts, the # characters are already doubled, because they're absorbed as the argument to a macro.
With the l3regex module of expl3 it's easier:
\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\allowhash}{m}
{
\tl_set:Nn \l_tmpa_tl { #1 }
\regex_replace_all:nnN { \cP\# } { \cO\# } \l_tmpa_tl
\tl_use:N \l_tmpa_tl
}
\ExplSyntaxOff
\begin{document}
\allowhash{One hash: #, not two}
\end{document}
You can easily add support for saving the token list in a macro.
\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\allowhash}{om}
{
\tl_set:Nn \l_tmpa_tl { #2 }
\regex_replace_all:nnN { \cP\# } { \cO\# } \l_tmpa_tl
\IfNoValueTF { #1 }
{ \tl_use:N \l_tmpa_tl }
{ \tl_set_eq:NN #1 \l_tmpa_tl }
}
\ExplSyntaxOff
\begin{document}
\allowhash{One hash: #, not two}
\allowhash[\foo]{One hash: #, not two}
\texttt{\meaning\foo}
\end{document}
(There seems to be a double space in the picture, but it's only due to \nonfrenchspacing.)
Let's look at your code:
\documentclass{article}
\makeatletter
\def\allowhash#1{
{\toks@{#1}\catcode`\#11\relax\scantokens\expandafter{\the\toks@}}%
}
\makeatother
\begin{document}
\allowhash{One hash: #, not two}
\end{document}
LaTeX does fetch an argument for \allowhash, hereby tokenizing the tokens which form that argument under normal category-code-régime. Thus LaTeX does fetch an explicit-hash-character-token of category code 6(parameter) as a component of \allowhash's argument.
When expanding \allowhash, this token becomes a part of the content of \toks@.
Thus \toks@ contains an explicit-hash-character-token of category code 6(parameter).
Due to \expandafter...\the-trickery this explicit-hash-character-token of category code 6(parameter) ends up as a component of the ⟨general text⟩ of \scantokens.
\scantokens emulates unexpanded-writing the tokens from its ⟨general text⟩ into file and reading them back from the file and hereby tokenizing things under the current category-code-régime.
When writing to file or screen, explicit character tokens of category-code 6(parameter) get doubled.
Thus the hash gets doubled by \scantokens unexpanded-writing-part.
Off the cuff I can only offer a routine \ReplaceEveryHash which takes one argument and does replace each explicit catcode-6-character-token of the argument by its stringification—this mechanism does not act only on explicit catcode-6-hashes but on all explicit catcode-6-character-tokens.
\documentclass{article}
\makeatletter
%%=============================================================================
%% Paraphernalia:
%% \UD@firstoftwo, \UD@secondoftwo,
%% \UD@PassFirstToSecond, \UD@Exchange, \UD@removespace
%% \UD@CheckWhetherNull, \UD@CheckWhetherBrace,
%% \UD@CheckWhetherLeadingSpace, \UD@ExtractFirstArg
%%=============================================================================
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\newcommand\UD@PassFirstToSecond[2]{#2{#1}}%
\newcommand\UD@Exchange[2]{#2#1}%
\newcommand\UD@removespace{}\UD@firstoftwo{\def\UD@removespace}{} {}%
%%-----------------------------------------------------------------------------
%% Check whether argument is empty:
%%.............................................................................
%% \UD@CheckWhetherNull{<Argument which is to be checked>}%
%% {<Tokens to be delivered in case that argument
%% which is to be checked is empty>}%
%% {<Tokens to be delivered in case that argument
%% which is to be checked is not empty>}%
%%
%% The gist of this macro comes from Robert R. Schneck's \ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
\newcommand\UD@CheckWhetherNull[1]{%
\romannumeral0\expandafter\UD@secondoftwo\string{\expandafter
\UD@secondoftwo\expandafter{\expandafter{\string#1}\expandafter
\UD@secondoftwo\string}\expandafter\UD@firstoftwo\expandafter{\expandafter
\UD@secondoftwo\string}\expandafter\expandafter\UD@firstoftwo{ }{}%
\UD@secondoftwo}{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@firstoftwo}%
}%
%%-----------------------------------------------------------------------------
%% Check whether argument's first token is a catcode-1-character
%%.............................................................................
%% \UD@CheckWhetherBrace{<Argument which is to be checked>}%
%% {<Tokens to be delivered in case that argument
%% which is to be checked has leading
%% catcode-1-token>}%
%% {<Tokens to be delivered in case that argument
%% which is to be checked has no leading
%% catcode-1-token>}%
\newcommand\UD@CheckWhetherBrace[1]{%
\romannumeral0\expandafter\UD@secondoftwo\expandafter{\expandafter{%
\string#1.}\expandafter\UD@firstoftwo\expandafter{\expandafter
\UD@secondoftwo\string}\expandafter\expandafter\UD@firstoftwo{ }{}%
\UD@firstoftwo}{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@secondoftwo}%
}%
%%-----------------------------------------------------------------------------
%% Check whether brace-balanced argument starts with a space-token
%%.............................................................................
%% \UD@CheckWhetherLeadingSpace{<Argument which is to be checked>}%
%% {<Tokens to be delivered in case <argument
%% which is to be checked>'s 1st token is a
%% space-token>}%
%% {<Tokens to be delivered in case <argument
%% which is to be checked>'s 1st token is not
%% a space-token>}%
\newcommand\UD@CheckWhetherLeadingSpace[1]{%
\romannumeral0\UD@CheckWhetherNull{#1}%
{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@secondoftwo}%
{\expandafter\UD@secondoftwo\string{\UD@CheckWhetherLeadingSpaceB.#1 }{}}%
}%
\newcommand\UD@CheckWhetherLeadingSpaceB{}%
\long\def\UD@CheckWhetherLeadingSpaceB#1 {%
\expandafter\UD@CheckWhetherNull\expandafter{\UD@secondoftwo#1{}}%
{\UD@Exchange{\UD@firstoftwo}}{\UD@Exchange{\UD@secondoftwo}}%
{\UD@Exchange{ }{\expandafter\expandafter\expandafter\expandafter
\expandafter\expandafter\expandafter}\expandafter\expandafter
\expandafter}\expandafter\UD@secondoftwo\expandafter{\string}%
}%
%%-----------------------------------------------------------------------------
%% Extract first inner undelimited argument:
%%
%% \UD@ExtractFirstArg{ABCDE} yields {A}
%%
%% \UD@ExtractFirstArg{{AB}CDE} yields {AB}
%%.............................................................................
\newcommand\UD@RemoveTillUD@SelDOm{}%
\long\def\UD@RemoveTillUD@SelDOm#1#2\UD@SelDOm{{#1}}%
\newcommand\UD@ExtractFirstArg[1]{%
\romannumeral0%
\UD@ExtractFirstArgLoop{#1\UD@SelDOm}%
}%
\newcommand\UD@ExtractFirstArgLoop[1]{%
\expandafter\UD@CheckWhetherNull\expandafter{\UD@firstoftwo{}#1}%
{ #1}%
{\expandafter\UD@ExtractFirstArgLoop\expandafter{\UD@RemoveTillUD@SelDOm#1}}%
}%
%%=============================================================================
%% \ReplaceEveryHash{<argument>}%
%%
%% Each explicit catcode-6(parameter)-character-token of the <argument>
%% will be replaced by its stringification.
%%
%% You obtain the result after two expansion-steps, i.e.,
%% in expansion-contexts you get the result after "hitting"
%% \ReplaceEveryHash by two \expandafter.
%%
%% As a side-effect, the routine does replace matching pairs of explicit
%% character tokens of catcode 1 and 2 by matching pairs of curly braces
%% of catcode 1 and 2.
%% I suppose this won't be a problem in most situations as usually the
%% curly braces are the only characters of category code 1 / 2...
%%
%% This routine needs \detokenize from the eTeX extensions.
%%-----------------------------------------------------------------------------
\newcommand\ReplaceEveryHash[1]{%
\romannumeral0\UD@ReplaceEveryHashLoop{#1}{}%
}%
\newcommand\UD@ReplaceEveryHashLoop[2]{%
\UD@CheckWhetherNull{#1}{ #2}{%
\UD@CheckWhetherLeadingSpace{#1}{%
\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@removespace#1}{#2 }%
}{%
\UD@CheckWhetherBrace{#1}{%
\expandafter\expandafter\expandafter\UD@PassFirstToSecond
\expandafter\expandafter\expandafter{%
\expandafter\UD@PassFirstToSecond\expandafter{%
\romannumeral0\expandafter\UD@ReplaceEveryHashLoop
\romannumeral0\UD@ExtractFirstArgLoop{#1\UD@SelDOm}{}%
}{#2}}%
{\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@firstoftwo{}#1}}%
}{%
\expandafter\UD@CheckWhetherHash
\romannumeral0\UD@ExtractFirstArgLoop{#1\UD@SelDOm}{#1}{#2}%
}%
}%
}%
}%
\newcommand\UD@CheckWhetherHash[3]{%
\expandafter\UD@CheckWhetherLeadingSpace\expandafter{\string#1}{%
\expandafter\expandafter\expandafter\UD@CheckWhetherNull
\expandafter\expandafter\expandafter{%
\expandafter\UD@removespace\string#1}{%
\expandafter\expandafter\expandafter\UD@CheckWhetherNull
\expandafter\expandafter\expandafter{%
\expandafter\UD@removespace\detokenize{#1}}{%
\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@firstoftwo{}#2}{#3#1}%
}{%
\expandafter\expandafter\expandafter\UD@PassFirstToSecond
\expandafter\expandafter\expandafter{\expandafter\UD@Exchange
\expandafter{\string#1}{#3}}{%
\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@firstoftwo{}#2}%
}%
}%
}{%
\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@firstoftwo{}#2}{#3#1}%
}%
}{%
\expandafter\expandafter\expandafter\UD@CheckWhetherNull
\expandafter\expandafter\expandafter{%
\expandafter\UD@firstoftwo
\expandafter{\expandafter}\string#1}{%
\expandafter\expandafter\expandafter\UD@CheckWhetherNull
\expandafter\expandafter\expandafter{%
\expandafter\UD@firstoftwo
\expandafter{\expandafter}\detokenize{#1}}{%
\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@firstoftwo{}#2}{#3#1}%
}{%
\expandafter\expandafter\expandafter\UD@PassFirstToSecond
\expandafter\expandafter\expandafter{\expandafter\UD@Exchange
\expandafter{\string#1}{#3}}{%
\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@firstoftwo{}#2}%
}%
}%
}{%
\expandafter\UD@ReplaceEveryHashLoop
\expandafter{\UD@firstoftwo{}#2}{#3#1}%
}%
}%
}%
%----------------------------------------------------------------------
\newcommand\allowhash[1]{\ReplaceEveryHash{#1}}
\newcommand\allowanddetokenizehash[1]{%
\detokenize\expandafter\expandafter\expandafter{\ReplaceEveryHash{#1}}%
}%
\makeatother
\begin{document}
\allowhash{One hash: #, not two}
\begingroup
\frenchspacing
\ttfamily \allowanddetokenizehash{One hash: #, not two. This time in braces:{#{#{#{#}}}#}}
\endgroup
For comparison the effect of \verb|\detokenize| without prior hash-replacing:
\begingroup
\frenchspacing
\ttfamily \detokenize{One hash: #, not two. This time in braces:{#{#{#{#}}}#}}
\endgroup
\end{document}
