Detect beginning of a sentence in a macro for capitalization

Question

Is it possible to make macros give different results at the beginning of a new sentence? Suppose that I want the macro "\secname" to write "Section" at the beginning of a new sentence, and "section" anywhere else. How can I do so?

Answer 1

You could set the \sfcode of the "end of sentence" chars to something different and test for it:

 \documentclass[10pt]{report}
 \sfcode`\.=1001
 \sfcode`\?=1001
 \sfcode`\!=1001
 \sfcode`\:=1001
 \newcommand\secname{\ifnum\spacefactor=1001 Secname\else secname\fi}
 \begin{document}
 abc. \secname\ is \secname.

 e.g.\@ \secname
 \end{document}

\nonfrenchspacing is also setting the \sfcodes. In this case you could use something like this:

 \documentclass[10pt]{report}
 \nonfrenchspacing
 \newcommand\secname{\ifnum\spacefactor>1900 Secname\else secname\fi}
 \begin{document}
 abc. \secname\ is \secname.
 abc: \secname, \secname.
 e.g.\@ \secname
 \end{document}

Answer 2

Easiest way is to define two macros:

\def\secname{section}
\def\Secname{Section}

It is a very difficult task to determine sentence boundaries and one of the hottest topics in Computational Linguistics. To do so properly you need to determine that in Dr. Who, for example, the period after the "Dr." does not end a sentence, so you need to parse for all abbreviations and when you think you test for the next letter to start with a capital letter, think of e.g. and all the Latin abbreviations we use.

Answer 3

Before Ulrike posted her nice answer that uses \spacefactor, I had thought this would be impossible in TeX without redefining .. Just for completeness: Here's my answer that does redefine . (after making it active, which probably is not such a good idea). Note that you do not have to use \@ as in Ulrike's solution.

\documentclass{article}
\let\period.
\catcode`.=\active
\let\qwe\relax
\futurelet\myspace{ }
\newcommand.{\period\futurelet\nextchar\testspace}
\newcommand\testspace{\ifx\nextchar\myspace\expandafter\eatspace\expandafter.\fi}
\def\eatspace. { \futurelet\nextchar\testsec}
\newcommand\testsec{\ifx\nextchar\secname\def\qwe{ }\fi}
\newcommand\secname{\ifx\qwe\relax section\else Section\let\qwe\relax\fi}
\begin{document}
abc. \secname\ is \secname.
abc: \secname, e.g.\ \secname.
\end{document}

Yes, This looks as if I had I tried to make it as obscure as possible. Two interesting points: 1. Note the definition of \myspace (\space does not work!). 2. I didn't manage to use LaTeX's \ifnextchar to test if the next character is a space, so I used \futurelet.

Answer 4

As an extension of the @YiannisLazarides answer, you can use mfirstuc which is included with TexLive 2014.

This will take care of the capitalisation for you, and you won't have to repeat yourself when defining the upper-case version of your command.

\documentclass{article}
\usepackage{mfirstuc}
\def\secname{fancy section}
\def\Secname{\expandafter\makefirstuc\expandafter{\secname}}

\begin{document}
Here's a new \secname{}. \Secname{} titles are good.
\end{document}

Which results in:

Answer 5

Here's a new answer, which is inspired in part by the recent query A macro that behaves differently at start of sentence, which got closed as duplicate of the present query (which I hadn't be aware of until today).

Suppose that the author has defined two macros called \agt, which outputs the string "the agent", and \Agt, which outputs "The agent" (with a capital "T"). In addition, let's suppose that the author would like \agt to behave like \Agt automatically if -- and only if -- \agt occurs at the start of a sentence. (For some reason, the author cannot manually replace \agt with \Agt as needed...)

Assuming we may safely dismiss cases such as Mr. \agt or Mrs. \agt as not likely to ever occur in a real document, the following LuaLaTeX-based solution may be of interest. It makes no assumptions about the \sfcode-status of various potential sentence-ending punctuation marks, and it works regardless of whether \frenchspacing or \nonfrenchspacing is in effect. Its working assumption is that \agt should be changed to \Agt under either of the following two circumstances:

• \agt occurs at the start of an input line. Whitespace between the start of the line and \agt is OK. Or,

• \agt is preceded by ?, !, or . (the three main sentence-ending punctuation characters in the English language), followed by one or more whitespace characters. (Thus, cases such as .\agt and ?\agt are not modified.)

  Naturally, if the language your document is written in features different sentence-ending punctuation characters, feel free to modify the list of characters enclosed by [...] in the second gsub statement shown below.

Aside: I wrote this answer assuming that people use an editor that "reflows" paragraphs by inserting "soft" line breaks. (That's the behavior of all editing software I employ...) However, as @jfbu has pointed out in a comment, other editors (including Emacs, apparently -- sorry, I don't use Emacs personally) use a different approach, viz., they reflow paragraphs by inserting "hard" line breaks, in effect storing a paragraph as a sequence of separate strings. Such a an approach clearly raises the risk that some instances of \agt will occur at the start of a line which, in turn, makes the proposed method much less attractive. Sigh. I'm afraid I don't know what advice to give.

Here, then, is an MWE.

% !TEX TS-program = lualatex
\documentclass{article}

\def\agt{the agent}
\def\Agt{The agent}

\usepackage{luacode}
\begin{luacode}
function agt_to_Agt ( s )
  s = s:gsub ( "^%s-\\agt", "\\Agt" )
  return s:gsub ( "([?!.])%s+\\agt" , "%1 \\Agt" )
end
\end{luacode}
\AtBeginDocument{\directlua{luatexbase.add_to_callback 
  ( "process_input_buffer" , agt_to_Agt , "agt_to_Agt" )}}
  
\begin{document}
\noindent
 \agt\ says x. \agt\ shouts Y!  \agt\ asks z? \agt\ whispers something 
to \agt.  \agt\ says the phrase,  ``\agt''.
 
\end{document}