How to define a command that bolds the first letter of each word in the input argument?

Question

I want to define a command that takes a sentence, a word or several words as input and bolds the first letter of each word. I need this mostly for abbreviations and so far I have done it manually in this way:

\documentclass[a4paper]{article} 

\begin{document} 
GNU is a recursive acronym for ``\emph{\bf{G}\normalfont{nu's} \bf{N}\normalfont{ot} \bf{U}\normalfont{nix}}'', chosen because GNU's design is Unix-like, but differs from Unix by being free software and containing no Unix code.
\end{document}

What I want to do, is to define a command like \BoldAbbrv that does the job. In other words:

\documentclass[a4paper]{article} 
%\newcommand{\BoldAbbrv}...IMPLEMENTATION
\begin{document} 
GNU is a recursive acronym for ``\BoldAbbrv{Gnu's Not Unix}'', chosen because GNU's design is Unix-like, but differs from Unix by being free software and containing no Unix code.
\end{document}

How can I do that? Moreover, a related question that came to my mind while thinking about this problem, is there such concept as wildcard in latex (e.g. "*" as zero or more characters and "_" as a single one)?

Answer 1

Here's an idea using expl3:

\documentclass{article}
\usepackage[T1]{fontenc}

\usepackage{expl3,xparse}

% turn expl3 space on: `:' and `_' are letters now and spaces
% are ignored. To insert a space use `~'.
\ExplSyntaxOn
% declare a new sequence variable:
\seq_new:N \l_pouya_boldfirst_seq

% the internal command:
\cs_new:Npn \pouya_boldfirst:n #1
  {
    % split the input at every space and put the items in the sequence:
    \seq_set_split:Nnn \l_pouya_boldfirst_seq { ~ } { #1 }
    % map over every item of the sequence; each item is referred to with
    % `##1' as we're inside of a command definition:
    \seq_map_inline:Nn \l_pouya_boldfirst_seq
      {
        % use the head of the item in the argument of \textbf
        % and put in the tail of the item after it
        % followed by a space:
        \textbf { \tl_head:n { ##1 } } \tl_tail:n { ##1 } ~
      }
    % undo the last space:
    \unskip
  }

% the document command:
\NewDocumentCommand\BoldFirst{m}
  { \pouya_boldfirst:n { #1 } }

% turn expl3 space off again:
\ExplSyntaxOff

\begin{document}

GNU is a recursive acronym for ``\BoldFirst{Gnu's Not Unix}'', chosen because GNU's
design is Unix-like, but differs from Unix by being free software and containing
no Unix code.

\end{document}

Answer 2

This seems to be a good application of the RegEx features of LaTeX3:

\documentclass{article}
\usepackage{xparse,l3regex}
\ExplSyntaxOn
\NewDocumentCommand{\BoldFirst}{ m }
 {
  \pouya_boldfirst:n { #1 }
 }
\cs_new_protected:Npn \pouya_boldfirst:n #1
 {
  \tl_set:Nn \l_pouya_input_tl { #1 }
  \regex_replace_once:nnN { \A(.*?[A-Za-z]) } { \c{textbf}\cB\{ \1 \cE\} } \l_pouya_input_tl
  \regex_replace_all:nnN { (\s) (.*?[A-Za-z]) } { \1  \c{textbf}\cB\{ \2 \cE\} } \l_pouya_input_tl
  \tl_use:N \l_pouya_input_tl
 }
\ExplSyntaxOff

\begin{document}
\BoldFirst{Gnu's Not Unix}

\BoldFirst{Gnu's Not \'Unix}

\BoldFirst{Gnu's Not \v{U}nix}
\end{document}

Also accented characters work, provided the LICR is used (no UTF-8 input).

Answer 3

A TeX solution using delimited arguments.

Macros:

• \BoldAbbrv{<text>} that does not expand its argument, and
• \eBoldAbbrv{<text>} that does expand its argument before processing.

The \BoldAbbrv macro suffice if you only use plain text in its argument.

Check the output of \BoldAbbrv{\GNU} and \eBoldAbbrv{\GNU} for the difference, but also the different ways how accents are handled (they need at least to be enclosed in braces).

Code

\documentclass[a4paper]{article}
\makeatletter
\newcommand*{\BoldAbbrv}[1]{%
 \qrr@BoldAbbrv#1 \relax
}
\newcommand*{\eBoldAbbrv}[1]{%
 \edef\qrr@BoldAbbrv@Arg{#1 }%
 \expandafter\qrr@BoldAbbrv\qrr@BoldAbbrv@Arg\relax
}
\def\qrr@BoldAbbrv#1 #2\relax{%
 \textbf#1\relax
 \ifx\relax#2\else
  \space\qrr@BoldAbbrv#2\relax
 \fi
}
\makeatother
\newcommand*{\GNU}{Gnu's Not Unix}
\begin{document}
\BoldAbbrv{Gnu's Not Unix} --- \BoldAbbrv{Gnu's Not {\"U}nix}

\BoldAbbrv{\GNU} --- \eBoldAbbrv{\GNU}

\BoldAbbrv{Gnu's Not {\d{U}}nix} --- \eBoldAbbrv{Gnu's Not {\noexpand\d{U}}nix}
\end{document}

Output

Answer 4

Already three answers and one accepted, but let me try the following code. It does allow some flexibility in the user input, but certainly will probably fail against more treacherous situations.

Update to deal with line endings... (previous version did not do the expected thing; attached image of the output updated to show now it works.)

Update so that a space after the opening brace is significant (in the previous version two or more were significant and counted as one, but a single one was ignored).

I have also illustrated the limitations of the idea. Quite probably, this is not the good way to go for this problem.

Final update: I have improved bits and pieces. See the output below.

\documentclass[a4paper]{article} 
%\newcommand{\BoldAbbrv}...IMPLEMENTATION

\usepackage{iftex}
\ifXeTeX\usepackage{fontspec}\fi
\ifLuaTeX\usepackage{fontspec}\fi
\ifPDFTeX\usepackage[utf8]{inputenc}\usepackage[T1]{fontenc}\fi

\def\LongJump#1#2#3#4{#2#3#4#1}
\makeatletter
{
\obeyspaces\catcode`\^^M\active\relax%
%
\gdef\BoldAbbrv@sp{\ifx\BoldAbbrv@token\bgroup%
\ \expandafter\textbf\else%
\ifx\BoldAbbrv@token\egroup\ \else%
\expandafter\expandafter\expandafter\BoldAbbrv@aux\fi\fi}%
%
\gdef\BoldAbbrv@eol{\ifx\BoldAbbrv@token\bgroup%
\ \expandafter\textbf\else%
\ifx\BoldAbbrv@token\egroup\ \else%
\expandafter\expandafter\expandafter\BoldAbbrv@auxeol\fi\fi}%
%
\gdef\BoldAbbrv@aux#1{\ifx#1 \expandafter \else%
\ifx#1^^M\expandafter\expandafter\expandafter \else%
\ifcat\noexpand#1a{\ \bfseries#1}\else%
\LongJump{\ #1}\fi\fi\fi}%
%
\gdef\BoldAbbrv@auxeol#1{\ifx#1 \expandafter^^M\else%
\ifx#1^^M\expandafter\expandafter\expandafter\BoldAbbrv@initpar\else%
\ifcat\noexpand#1a{\ \bfseries#1}\else%
\LongJump{\ #1}\fi\fi\fi}%
%
\gdef\BoldAbbrv@inita{\def {\futurelet\BoldAbbrv@token\BoldAbbrv@sp}%
\def^^M{\futurelet\BoldAbbrv@token\BoldAbbrv@eol}}%
%
\gdef\BoldAbbrv@auxb#1{\ifx#1 \expandafter \else%
\ifx#1^^M\expandafter\expandafter\expandafter \else%
\ifcat\noexpand#1a{\bfseries#1}\else%
\LongJump{#1}\fi\fi\fi}%
%
\gdef\BoldAbbrv@nosp{\ifx\BoldAbbrv@token\bgroup%
\expandafter\textbf\else%
\ifx\BoldAbbrv@token\egroup\else%
\expandafter\expandafter\expandafter\BoldAbbrv@auxb\fi\fi}%
%
\gdef\BoldAbbrv@initb{\futurelet\BoldAbbrv@token\BoldAbbrv@nosp}%
}
%
\def\BoldAbbrv@initpar{\par\BoldAbbrv@initb}
\def\BoldAbbrv#1#{\bgroup\obeyspaces\catcode`\^^M=\active
     \BoldAbbrv@inita\afterassignment\BoldAbbrv@initb\let\next= }
\makeatother

\begin{document}\thispagestyle{empty}
GNU is a recursive acronym for ``\BoldAbbrv{Gnu's     Not Unix}'', chosen
because GNU's design is Unix-like, but differs from Unix by being free
software and containing no Unix code.

Space Torture test (\emph{n.b.} spaces or endlines just after 
the opening brace
or just
before the closing one count as one):
GNU is a recursive acronym for ``\BoldAbbrv{Gnu's    Not      Unix'', 
}%
\BoldAbbrv    {    
chosen
because GNU's design      is Unix-like, but   differs from Unix 
                by being free
software     and containing     }no Unix code.

This new version does not bolden quotes, unfortunately it
doesn't either bolden the letter after the quote(s): \par
\BoldAbbrv{GNU is a recursive acronym for ``Gnu's Not Unix''}.

\BoldAbbrv{Things put {inside braces} in the argument will be
  typeset {entirely bold.}

Accents need to be put inside braces. Omitting
  braces does \emph{not} create an error. UTF8 letters in
  PDFLaTeX need to be put inside braces for the macro to
  operate, but this is not necessary with XeTeX. And in both
  cases no compilation errors when braces are omitted.
  Examples: Gnu's Not Unix --- Gnu's Not {\"U}nix (ok) ---
  Gnu's Not \"Unix (no) --- Gnu's Not {Ü}nix (ok) --- Gnu's
  Not Ünix (ok with Unicode engines). As ok followed a
  parenthesis it was not boldened. The parenthesis not being a
  letter wasn't boldened either.}

The new version has no effect on a macro even if it expands to
a string:
\newcommand*{\GNU}{Gnu's Not Unix}\BoldAbbrv{\GNU}.
This is sad but the new version is compatible with more
general input. It only boldens letters, or groups enclosed
within braces. \BoldAbbrv{{\GNU}}.

\BoldAbbrv{The macro {can} be applied to more than
  one paragraph simultaneously. Yes, really.

The macro {can} be applied to more than
  one paragraph simultaneously. Yes, really.

\BoldAbbrv{And it \BoldAbbrv{can be} nested. Although one does
not see the interest.}
}

An empty argument\BoldAbbrv{} is no error.

A space after the opening brace or before the closing brace
counts: \BoldAbbrv{ Gnu's Not {\"U}nix }X. Two or more are like
just one:\BoldAbbrv{   Gnu's Not {\"U}nix    }X.

This does not quite work: \BoldAbbrv{\emph{An emphasized
    argument.}} And this neither: \BoldAbbrv{\itshape{}An
  italicized argument,\/} where we got rid of the initial
superfluous space which is in this example:
\BoldAbbrv{\itshape An italicized argument with a spurious
  initial space.} So one must do {\itshape\BoldAbbrv{This is
    not argument to a macro, and it works.}\/}

\textsl{\BoldAbbrv {The macro does not work in the argument to
    another macro (except itself... as it is not really a
    macro with argument despite appearances!)}}

\end{document}