Test if the first character of a string is 'a'

Question

Is there an easy way to make a macro returning a string (e.g. 'true') if the string input begins by some fixed character, say 'a' ? I can't figure how to combine the different packages to do this.

\testchain{a} will return the string 'true' and nothing otherwise.

Answer 1

The xstring package offers plenty of useful commands for string manipulation. However, if you're worried about performance for a very specific task, you may want to use lower-level commands, as in egreg's and Qrrbrbirlbel's answers.

Here's a solution using xstring; note that the argument to \testchain is fully expanded, if any expansion can be applied.

\documentclass{article}

\usepackage{xstring}

\newcommand\testchain[1]{%
  \StrLeft{#1}{1}[\firstchar]%
  \IfStrEq{\firstchar}{a}{true}{false}%
}

\begin{document}

\noindent
    Test 1: \testchain{adfgi7634r}\\
    Test 2: \testchain{sdfkhsdf}\\  
    \def\abc{abc}
    \def\ghi{ghi}
    Test 3: \testchain{\abc}\\
    Test 4: \testchain{\ghi}
\end{document}

Answer 2

This is an expandable test, which means it can be used in an \edef as shown in the last line:

\documentclass{article}
\usepackage{xparse}

\makeatletter
\newcommand\testchain[1]{%
  \ifnum\pdfstrcmp{\unexpanded\expandafter{\@car#1\@nil}}{a}=\z@
    \expandafter\@firstofone
  \else
    \expandafter\@gobble
  \fi{true}%
}
\def\testchain@a{a}
\makeatother

\begin{document}
X\testchain{a}X

X\testchain{abc}X

X\testchain{x}X

\edef\exptest{\testchain{abc}}\texttt{\meaning\exptest}
\end{document}

It uses \pdfstrcmp and \unexpanded, so it's not available in "classic TeX". If you plan to use the macro with XeLaTeX or LuaLaTeX, then load the package pdftexcmds and use \@pdfstrcmp instead of \pdfstrcmp.

---

Here is a LaTeX3 version.

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\testchain}{m}
 {
  \testchain_main:n { #1 }
 }
\cs_new:Npn \testchain_main:n #1
 {
  \str_if_eq_x:nnT { \tl_head:n { #1 } } { a } { true }
 }
\ExplSyntaxOff

\begin{document}
X\testchain{a}X

X\testchain{abc}X

X\testchain{x}X

\edef\exptest{\testchain{abc}}\texttt{\meaning\exptest}
\end{document}

A variant that allows to use a string buried into a macro, by calling \testchain*:

\documentclass{article}
\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\testchain}{sm}
 {
  \IfBooleanTF{#1}
   { \testchain_main:o { #2 } }
   { \testchain_main:n { #2 } }
 }
\cs_new:Npn \testchain_main:n #1
 {
  \str_if_eq_x:nnT { \tl_head:n { #1 } } { a } { true }
 }
\cs_generate_variant:Nn \testchain_main:n { o }
\ExplSyntaxOff

\begin{document}
X\testchain{a}X

X\testchain{abc}X

X\testchain{x}X

\def\stringI{abc}
X\testchain{\stringI}X

% show it's fully expandable
\edef\exptest{\testchain*{\stringI}}\texttt{\meaning\exptest}
\end{document}

Answer 3

I present a Lua solution which works with all macro packages, including plainTeX, ConTeXt and LaTex. It defines a simple Lua funktion testing for the first character. Then a TeX macro is defined calling the Lua funktion.

\def\beginsWith#1#2#3%
  {\directlua{userdata.beginsWith([===[#1]===],[===[#2]===],[===[#3]===])}}

\directlua{
  userdata = userdata or {}
  userdata.beginsWith = function(str, arg2, arg3)
    if string.find(str, "^a") then
      tex.print(arg2)
    else
      tex.print(arg3)
    end
  end}

\beginsWith{abcde}{true}{false}\par
\beginsWith{zyxwv}{true}{false}
\bye

The output:

true
false

Answer 4

\documentclass{article}
\makeatletter
\newcommand*{\eifstartswith}{\@expandtwoargs\ifstartswith}
\newcommand*{\ifstartswith}[2]{%
  \if\@car#1.\@nil\@car#2.\@nil
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi}
\makeatother
\begin{document}
\ifstartswith{a}{abba}{true}{false}

\ifstartswith{b}{abba}{true}{false}

\newcommand*{\stringA}{a}
\newcommand*{\stringB}{b}
\newcommand*{\stringC}{abba}

\eifstartswith{\stringA}{\stringC}{true}{false}

\eifstartswith{\stringB}{\stringC}{true}{false}
\end{document}

Answer 5

The macro \isnextbyte of the stringstrings package compares its first [single-byte] argument to the first byte of its second argument. It will print a T or F unless invoked in quiet mode, in which case it stores its result in \theresult. The following MWE will produce a T, F, T, and null result.

\documentclass{article}
\usepackage{stringstrings}
\begin{document}
\isnextbyte{a}{another test}\par
\isnextbyte{a}{but not this one}\par

This will not print F for false:\par
\isnextbyte[q]{a}{and this is in quiet mode}
\if T\theresult T\fi
\isnextbyte[q]{a}{false result}
\if T\theresult T\fi
\end{document}%