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Some programming editors and IDEs highlight legal and illegal variable names in different ways. I wish listings easily allowed for such syntax checking and highlighting, but it doesn't, at least at the moment. My longer-term objective is to implement a solution compatible with listings but, for now, I'm looking at a very simplified version of the problem.

I want to parse the first word in a token stream composed only of character and space tokens. This has to be done token by token, though, because I have to perform some checks on each token. For that, I use a recursive macro that parses the stream, saving it in a \Word macro, until a . token is encountered. At that stage, I process \Word in a certain way, such as printing it in red. I define a word as a sequence of character tokens uninterrupted by any space token.

Problem: I only use a . token here because I can't figure out what I should change in my code in order to stop the recursion when the next space token, not a . token, is encountered in the stream. Substituting a control space (\) for . doesn't seem to do the trick. What should I change in my code?

I favour a solution using low-level TeX commands for the parsing, but LaTeX2e and LaTeX3 alternatives are also of interest to me.

Edit: I apologise if I seem to be moving the goalpost, but I had to clarify my question quite a bit by adding the "token by token" requirement, which may invalidate some of the answers already posted.

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\documentclass{article} \usepackage{xcolor} \def\ParseWordandPrint#1{% \if #1.% \textcolor{red}{\Word}\ % \else \edef\Word{\Word#1} \expandafter\ParseWordandPrint \fi% } \def\InitParseWordandPrint#1{% \def\Word{} \ParseWordandPrint#1% } \begin{document} \InitParseWordandPrint Hello.World \end{document}
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  • You can surely add the token by token processing when you have the various items stored in a sequence. It mostly depends on what you want to do during this processing. Commented Dec 21, 2013 at 21:43
  • Use Lua from LuaTeX. Commented Dec 22, 2013 at 11:36

3 Answers 3

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Undeleting this in light of your edited question.

It is easier to grab the word in one go with a space delimited argument and then iterate over that letter by letter. (As an example I check here that they are letters, the last example produces

illegal character 0 illegal character 1

To iterate character by character stopping at a space you tent to have to use \futurelet (or equivalently \@ifnextchar) but taht's not so good in a word you are going to typeset as it's hard not to break inter-letter kerns and ligatures. So it is easier to grab the word first.

\documentclass{article} \usepackage{xcolor} \def\InitParseWordandPrint#1 {\check#1\relax\textcolor{red}{#1} } \def\check#1{% \ifx\relax#1% \else \ifcat a#1% \else \typeout{illegal character #1}% \fi \expandafter\check \fi} \begin{document} \InitParseWordandPrint Hello World \InitParseWordandPrint World Hello World \InitParseWordandPrint W0r1d \end{document}
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  • Thanks, David. I think you deserve the checkmark because I was more in favour of a TeX solution. Commented Dec 22, 2013 at 13:29
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TeX's \def provides delimited parameter text. So, in this case, you can use the space as a delimiter:

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\documentclass{article} \usepackage{xcolor} \def\ParseWordandPrint#1{% \if #1.% \textcolor{red}{\Word}\ % \else% \edef\Word{\Word#1}% \expandafter\ParseWordandPrint% \fi% } \def\InitParseWordandPrint#1{% \def\Word{}% \ParseWordandPrint#1% } \def\highlightfirst#1 {\textcolor{red}{#1} } \begin{document} \InitParseWordandPrint Hello.World \highlightfirst Hello World. \end{document}
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  • Your approach solves my simplified problem. However, I really need to parse a word token by token in order to check whether it be a legal variable name or not, and I don't think I can use your approach for that. I've clarified my question. Commented Dec 21, 2013 at 20:51
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It's really easy with LaTeX3:

\documentclass{article} \usepackage{xparse,xcolor} \ExplSyntaxOn \NewDocumentCommand{\printfirstwordincolor}{ O{red} m } { \jubobs_pfwr:nn { #1 } { #2 } } \seq_new:N \l_jubobs_words_seq \tl_new:N \l_jubobs_first_word_tl \cs_new_protected:Npn \jubobs_pfwr:nn #1 #2 { % split the input at spaces \seq_set_split:Nnn \l_jubobs_words_seq { ~ } { #2 } % pop off the leftmost item \seq_pop_left:NN \l_jubobs_words_seq \l_jubobs_first_word_tl % print the first item in the chosen color \textcolor{#1}{ \l_jubobs_first_word_tl } ~ % % print the other items adding spaces between them \seq_use:Nn \l_jubobs_words_seq { ~ } } \ExplSyntaxOff \begin{document} \printfirstwordincolor{Hello World} \printfirstwordincolor[green]{Addio mondo crudele} \end{document}

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If you also want to process the input token by token, you can do a mapping on the saved items. Let's say you want to capitalize every ‘d’:

\documentclass{article} \usepackage{xparse,xcolor} \ExplSyntaxOn \NewDocumentCommand{\printfirstwordincolor}{ O{red} m } { \jubobs_pfwc:nn { #1 } { #2 } } \seq_new:N \l_jubobs_words_seq \tl_new:N \l_jubobs_first_word_tl \bool_new:N \l_jubobs_first_item_bool \cs_new_protected:Npn \jubobs_pfwc:nn #1 #2 { \seq_set_split:Nnn \l_jubobs_words_seq { ~ } { #2 } \seq_pop_left:NN \l_jubobs_words_seq \l_jubobs_first_word_tl \textcolor{#1}{ \l_jubobs_first_word_tl } ~ % \seq_use:Nn \l_jubobs_words_seq { ~ } } \NewDocumentCommand{\printfirstwordincolorandcapitalizeD} { O{red} m } { \jubobs_pfwcacd:nn { #1 } { #2 } } \cs_new_protected:Npn \jubobs_pfwcacd:nn #1 #2 { \seq_set_split:Nnn \l_jubobs_words_seq { ~ } { #2 } \leavevmode \bool_set_true:N \l_jubobs_first_item_bool \seq_map_inline:Nn \l_jubobs_words_seq { \bool_if:NT \l_jubobs_first_item_bool { \c_group_begin_token \color{#1} } \tl_map_inline:nn { ##1 } { \peek_charcode_remove:NT d { D } ####1 } \bool_if:NT \l_jubobs_first_item_bool { \c_group_end_token \bool_set_false:N \l_jubobs_first_item_bool } \c_space_tl } \unskip } \ExplSyntaxOff \begin{document} \printfirstwordincolor{Hello World} \printfirstwordincolorandcapitalizeD[blue]{Addio mondo crudele} \end{document}

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  • I'm still not very familiar with LaTeX3 syntax, but the comments you left in your code will be helpful. Thanks for this LaTeX3 alternative. Commented Dec 21, 2013 at 18:48

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