Test if an argument starts with a particular string

Question

Is there a way in tex to test if an argument starts with some string or not?
And then extract the text that follows that start?

As an example, I want to define a command that makes a text bold if it starts with my and italic otherwise.

\transform{my big world}
% Returns "\textbf{big world}". 

\transform{our small world}
% Returns "\textit{small world}". 

\transform{your world}
% Returns \textit{world}

% It would be great if the test is case insensitive 
\transform{My world} 
% Returns \textbf{world}

If possible, I prefer a method not using expl3 (I don't understand it enough to tinker with it).

The answers here gobbles a single character, which I can use for every character of my string. But that would get unwieldy rather quickly for long strings.

Answer 1

If your first word is always separated from the rest by a space, you can do with the following code:

\documentclass{article}

\makeatletter
\newcommand{\transform}[1]{\transform@#1\@nil}
\def\transform@@prefix{my}
\def\transform@#1 #2\@nil{%
  \lowercase{\def\transform@@start{#1}}%
  \ifx\transform@@start\transform@@prefix
    \textbf{#2}%
  \else
    \textit{#2}%
  \fi
}
\makeatother

\begin{document}

\transform{my big world} should return \textbf{big world}.

\transform{our small world} should return \textit{small world}.

\transform{your world} should return \textit{world}

\transform{My world} should return \textbf{world}

\end{document}

The first word is isolated and its lowercase version is matched with my.

Since you want a colon, rather than a space, it's possible to manage the absence of a prefix.

\documentclass{article}

\makeatletter
\newcommand{\transform}[1]{\transform@#1::\@nil}
\def\transform@@prefix{my}
\def\transform@#1:#2:#3\@nil{%
  \if\relax\detokenize{#2}\relax
    % no colon in #1
    \textit{#1}%
  \else
    \lowercase{\def\transform@@start{#1}}%
      \ifx\transform@@start\transform@@prefix
        \textbf{#2}%
      \else
        \textit{#2}%
      \fi
  \fi
}
\makeatother

\begin{document}

\transform{my:big world} should return \textbf{big world}.

\transform{our:small world} should return \textit{small world}.

\transform{your:world} should return \textit{world}

\transform{My:world} should return \textbf{world}

\transform{world} should return \textit{world}

\end{document}

An expl3 version, which is also (almost) fully expandable (it is not really because of \textit and \textbf, but we can use \text_expand:n to take care of that.

\documentclass{article}

\ExplSyntaxOn

% just for the final test
\cs_new_eq:NN \textexpand \text_expand:n
%%%

\NewExpandableDocumentCommand{\transform}{sm}
 {
  \IfBooleanTF{#1}
   {
    \tohiko_transform:V #2
   }
   {
    \tohiko_transform:n { #2 }
   }
 }

\cs_new:Npx \tohiko_transform:n #1
 {% colons are special in expl3
  \exp_not:N \__tohiko_transform:w #1 \c_colon_str \c_colon_str \exp_not:N \q_stop
 }
\cs_generate_variant:Nn \tohiko_transform:n { V }

\use:x
 {% colons are special in expl3
  \cs_new:Npn \exp_not:N \__tohiko_transform:w 
    ##1 \c_colon_str ##2 \c_colon_str ##3 \exp_not:N \q_stop
 }
 {
  \tl_if_empty:nTF { #2 }
   {
    \__tohiko_transformed:Nn \textit { #1 }
   }
   {
    \__tohiko_transform:nn { #1 } { #2 }
   }
 }

\cs_new:Npn \__tohiko_transform:nn #1 #2
 {
  \str_if_eq:eeTF { \str_lowercase:n { #1 } } { my }
   {
    \__tohiko_transformed:Nn \textbf { #2 }
   }
   {
    \__tohiko_transformed:Nn \textit { #2 }
   }
 }

\cs_new:Npn \__tohiko_transformed:Nn #1 #2
 {
  #1{\tl_trim_spaces:n{#2}}
 }

\ExplSyntaxOff

\newcommand{\testA}{my:hello world}
\newcommand{\testB}{our:hello world}

\begin{document}

\transform{my: big world} should return \textbf{big world}.

\transform{our:small world} should return \textit{small world}.

\transform{your:world } should return \textit{world}.

\transform{My:world} should return \textbf{world}.

\transform{world} should return \textit{world}.

\transform*{\testA} should return \textbf{hello world}.

\transform*{\testB} should return \textit{hello world}.

\edef\test{\textexpand{\transform{my: big world}}}
\texttt{\meaning\test}

\edef\test{\textexpand{\transform*{\testB}}}
\texttt{\meaning\test}

\end{document}