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+% \iffalse meta-comment
+%
+%% File: l3str.dtx Copyright (C) 2011-2014 The LaTeX3 Project
+%%
+%% It may be distributed and/or modified under the conditions of the
+%% LaTeX Project Public License (LPPL), either version 1.3c of this
+%% license or (at your option) any later version. The latest version
+%% of this license is in the file
+%%
+%% http://www.latex-project.org/lppl.txt
+%%
+%% This file is part of the "l3experimental bundle" (The Work in LPPL)
+%% and all files in that bundle must be distributed together.
+%%
+%% The released version of this bundle is available from CTAN.
+%%
+%% -----------------------------------------------------------------------
+%%
+%% The development version of the bundle can be found at
+%%
+%% http://www.latex-project.org/svnroot/experimental/trunk/
+%%
+%% for those people who are interested.
+%%
+%%%%%%%%%%%
+%% NOTE: %%
+%%%%%%%%%%%
+%%
+%% Snapshots taken from the repository represent work in progress and may
+%% not work or may contain conflicting material! We therefore ask
+%% people _not_ to put them into distributions, archives, etc. without
+%% prior consultation with the LaTeX3 Project.
+%%
+%% -----------------------------------------------------------------------
+%
+%<*driver|package>
+\RequirePackage{expl3}
+\GetIdInfo$Id: l3str-expl.dtx 5226 2014-07-18 18:08:01Z joseph $
+ {L3 Experimental strings}
+\def\ExplSyntaxName{l3str}
+%</driver|package>
+%<*driver>
+\documentclass[full]{l3doc}
+\usepackage{amsmath}
+\begin{document}
+ \DocInput{\jobname.dtx}
+\end{document}
+%</driver>
+% \fi
+%
+%
+% \title{^^A
+% The \textsf{l3str} package: manipulating strings of characters^^A
+% \thanks{This file describes v\ExplFileVersion,
+% last revised \ExplFileDate.}^^A
+% }
+%
+% \author{^^A
+% The \LaTeX3 Project\thanks
+% {^^A
+% E-mail:
+% \href{mailto:latex-team@latex-project.org}
+% {latex-team@latex-project.org}^^A
+% }^^A
+% }
+%
+% \date{Released \ExplFileDate}
+%
+% \maketitle
+%
+% \tableofcontents
+%
+% \begin{documentation}
+%
+% \LaTeX3 provides a set of functions to manipulate token lists
+% as strings of characters, ignoring the category codes of those
+% characters.
+%
+% String variables are simply specialised token lists, but by convention
+% should be named with the suffix \texttt{\ldots{}str}. Such variables
+% should contain characters with category code $12$ (other), except
+% spaces, which have category code $10$ (blank space). All the
+% functions in this module which accept a token list argument first
+% convert it to a string using \cs{tl_to_str:n} for internal processing,
+% and will not treat a token list or the corresponding string
+% representation differently.
+%
+% Most expandable functions in this module come in three flavours:
+% \begin{itemize}
+% \item \cs{str_...:N}, which expect a token list or string
+% variable as their argument;
+% \item \cs{str_...:n}, taking any token list (or string) as an
+% argument;
+% \item \cs{str_..._ignore_spaces:n}, which ignores any space
+% encountered during the operation: these functions are typically
+% faster than those which take care of escaping spaces
+% appropriately.
+% \end{itemize}
+%
+% \section{Building strings}
+%
+% \begin{variable}
+% {
+% \c_backslash_str,
+% \c_left_brace_str,
+% \c_right_brace_str,
+% \c_hash_str,
+% \c_tilde_str,
+% \c_percent_str
+% }
+% Constant strings, containing a single character token, with category
+% code $12$. Any character can be accessed as \cs{iow_char:N}
+% |\|\meta{character}.
+% \end{variable}
+%
+% \begin{function}{\str_new:N, \str_new:c}
+% \begin{syntax}
+% \cs{str_new:N} \meta{str~var}
+% \end{syntax}
+% Creates a new \meta{str~var} or raises an error if the name is
+% already taken. The declaration is global. The \meta{str~var} will
+% initially be empty.
+% \end{function}
+%
+% \begin{function}{\str_const:Nn, \str_const:Nx, \str_const:cn, \str_const:cx}
+% \begin{syntax}
+% \cs{str_const:Nn} \meta{str~var} \Arg{token list}
+% \end{syntax}
+% Creates a new constant \meta{str~var} or raises an error if the name
+% is already taken. The value of the \meta{str~var} will be set
+% globally to the \meta{token list}, converted to a string.
+% \end{function}
+%
+% \begin{function}
+% {
+% \str_set:Nn, \str_set:Nx, \str_set:cn, \str_set:cx,
+% \str_gset:Nn, \str_gset:Nx, \str_gset:cn, \str_gset:cx
+% }
+% \begin{syntax}
+% \cs{str_set:Nn} \meta{str var} \Arg{token list}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{string}, and stores the
+% result in \meta{str var}.
+% \end{function}
+%
+% \begin{function}
+% {
+% \str_put_left:Nn, \str_put_left:Nx,
+% \str_put_left:cn, \str_put_left:cx,
+% \str_gput_left:Nn, \str_gput_left:Nx,
+% \str_gput_left:cn, \str_gput_left:cx
+% }
+% \begin{syntax}
+% \cs{str_put_left:Nn} \meta{str var} \Arg{token list}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{string}, and prepends the
+% result to \meta{str var}. The current contents of the \meta{str
+% var} are not automatically converted to a string.
+% \end{function}
+%
+% \begin{function}
+% {
+% \str_put_right:Nn, \str_put_right:Nx,
+% \str_put_right:cn, \str_put_right:cx,
+% \str_gput_right:Nn, \str_gput_right:Nx,
+% \str_gput_right:cn, \str_gput_right:cx
+% }
+% \begin{syntax}
+% \cs{str_put_right:Nn} \meta{str var} \Arg{token list}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{string}, and appends the
+% result to \meta{str var}. The current contents of the \meta{str
+% var} are not automatically converted to a string.
+% \end{function}
+%
+% \section{Accessing the contents of a string}
+%
+% \begin{function}[EXP]
+% {\str_count:N, \str_count:n, \str_count_ignore_spaces:n}
+% \begin{syntax}
+% \cs{str_count:n} \Arg{token list}
+% \end{syntax}
+% Leaves in the input stream the number of characters in the string
+% representation of \meta{token list}, as an integer denotation. The
+% functions differ in their treatment of spaces. In the case of
+% \cs{str_count:N} and \cs{str_count:n}, all characters including
+% spaces are counted. The \cs{str_count_ignore_spaces:n} function
+% leaves the number of non-space characters in the input stream.
+% \end{function}
+%
+% \begin{function}[EXP]{\str_count_spaces:N, \str_count_spaces:n}
+% \begin{syntax}
+% \cs{str_count_spaces:n} \Arg{token list}
+% \end{syntax}
+% Leaves in the input stream the number of space characters in the
+% string representation of \meta{token list}, as an integer
+% denotation. Of course, this function has no \texttt{_ignore_spaces}
+% variant.
+% \end{function}
+%
+% \begin{function}[EXP]{\str_head:N, \str_head:n, \str_head_ignore_spaces:n}
+% \begin{syntax}
+% \cs{str_head:n} \Arg{token list}
+% \end{syntax}
+% Converts the \meta{token list} into a \meta{string}. The first
+% character in the \meta{string} is then left in the input stream,
+% with category code \enquote{other}. The functions differ if the
+% first character is a space: \cs{str_head:N} and \cs{str_head:n}
+% return a space token with category code~$10$ (blank space), while
+% the \cs{str_head_ignore_spaces:n} function ignores this space
+% character and leaves the first non-space character in the input
+% stream. If the \meta{string} is empty (or only contains spaces in
+% the case of the \texttt{_ignore_spaces} function), then nothing is
+% left on the input stream.
+% \end{function}
+%
+% \begin{function}[EXP]{\str_tail:N, \str_tail:n, \str_tail_ignore_spaces:n}
+% \begin{syntax}
+% \cs{str_tail:n} \Arg{token list}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{string}, removes the first
+% character, and leaves the remaining characters (if any) in the input
+% stream, with category codes $12$ and $10$ (for spaces). The
+% functions differ in the case where the first character is a space:
+% \cs{str_tail:N} and \cs{str_tail:n} will trim only that space, while
+% \cs{str_tail_ignore_spaces:n} removes the first non-space character
+% and any space before it. If the \meta{token list} is empty (or
+% blank in the case of the \texttt{_ignore_spaces} variant), then
+% nothing is left on the input stream.
+% \end{function}
+%
+% \begin{function}[EXP]{\str_item:Nn, \str_item:nn, \str_item_ignore_spaces:nn}
+% \begin{syntax}
+% \cs{str_item:nn} \Arg{token list} \Arg{integer expression}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{string}, and leaves in the
+% input stream the character in position \meta{integer expression} of
+% the \meta{string}, starting at $1$ for the first (left-most)
+% character. In the case of \cs{str_item:Nn} and \cs{str_item:nn},
+% all characters including spaces are taken into account. The
+% \cs{str_item_ignore_spaces:nn} function skips spaces when counting
+% characters. If the \meta{integer expression} is negative,
+% characters are counted from the end of the \meta{string}. Hence,
+% $-1$ is the right-most character, \emph{etc.}
+% \end{function}
+%
+% \begin{function}[EXP]
+% {\str_range:Nnn, \str_range:nnn, \str_range_ignore_spaces:nnn}
+% \begin{syntax}
+% \cs{str_range:nnn} \Arg{token list} \Arg{start index} \Arg{end index}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{string}, and leaves in the
+% input stream the characters from the \meta{start index} to the
+% \meta{end index} inclusive. Positive \meta{indices} are counted
+% from the start of the string, $1$~being the first character, and
+% negative \meta{indices} are counted from the end of the string,
+% $-1$~being the last character. If either of \meta{start index} or
+% \meta{end index} is~$0$, the result is empty. For instance,
+% \begin{verbatim}
+% \iow_term:x { \str_range:nnn { abcdef } { 2 } { 5 } }
+% \iow_term:x { \str_range:nnn { abcdef } { -4 } { -1 } }
+% \iow_term:x { \str_range:nnn { abcdef } { -2 } { -1 } }
+% \iow_term:x { \str_range:nnn { abcdef } { 0 } { -1 } }
+% \end{verbatim}
+% will print \texttt{bcd}, \texttt{cdef}, \texttt{ef}, and an empty
+% line to the terminal.
+% \end{function}
+%
+% \section{String conditionals}
+%
+% \begin{function}[EXP,pTF]
+% {
+% \str_if_eq:nn, \str_if_eq:Vn, \str_if_eq:on, \str_if_eq:no,
+% \str_if_eq:nV, \str_if_eq:VV
+% }
+% \begin{syntax}
+% \cs{str_if_eq_p:nn} \Arg{tl_1} \Arg{tl_2}
+% \cs{str_if_eq:nnTF} \Arg{tl_1} \Arg{tl_2} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Compares the string representations of the two \meta{token lists} on
+% a character by character basis, and is \texttt{true} if the two
+% lists contain the same characters in the same order. Thus for
+% example
+% \begin{verbatim}
+% \str_if_eq_p:no { abc } { \tl_to_str:n { abc } }
+% \end{verbatim}
+% is logically \texttt{true}.
+% \end{function}
+%
+% \begin{function}[EXP,pTF, added = 2012-06-05]{\str_if_eq_x:nn}
+% \begin{syntax}
+% \cs{str_if_eq_x_p:nn} \Arg{tl_1} \Arg{tl_2}
+% \cs{str_if_eq_x:nnTF} \Arg{tl_1} \Arg{tl_2} \Arg{true code} \Arg{false code}
+% \end{syntax}
+% Compares the full expansion of two \meta{token lists} on a character by
+% character basis, and is \texttt{true} if the two lists contain the same
+% characters in the same order. Thus for example
+% \begin{verbatim}
+% \str_if_eq_x_p:nn { abc } { \tl_to_str:n { abc } }
+% \end{verbatim}
+% is logically \texttt{true}.
+% \end{function}
+%
+% \begin{function}[added = 2013-07-24, EXP, TF]{\str_case:nn, \str_case:on}
+% \begin{syntax}
+% \cs{str_case:nnTF} \Arg{test string} \\
+% ~~|{| \\
+% ~~~~\Arg{string case_1} \Arg{code case_1} \\
+% ~~~~\Arg{string case_2} \Arg{code case_2} \\
+% ~~~~\ldots \\
+% ~~~~\Arg{string case_n} \Arg{code case_n} \\
+% ~~|}| \\
+% ~~\Arg{true code}
+% ~~\Arg{false code}
+% \end{syntax}
+% This function compares the \meta{test string} in turn with each
+% of the \meta{string cases}. If the two are equal (as described for
+% \cs{str_if_eq:nnTF} then the
+% associated \meta{code} is left in the input stream. If any of the
+% cases are matched, the \meta{true code} is also inserted into the
+% input stream (after the code for the appropriate case), while if none
+% match then the \meta{false code} is inserted. The function
+% \cs{str_case:nn}, which does nothing if there is no match, is also
+% available.
+% \end{function}
+%
+% \begin{function}[added = 2013-07-24, EXP, TF]{\str_case_x:nn}
+% \begin{syntax}
+% \cs{str_case_x:nnn} \Arg{test string} \\
+% ~~|{| \\
+% ~~~~\Arg{string case_1} \Arg{code case_1} \\
+% ~~~~\Arg{string case_2} \Arg{code case_2} \\
+% ~~~~\ldots \\
+% ~~~~\Arg{string case_n} \Arg{code case_n} \\
+% ~~|}| \\
+% ~~\Arg{true code}
+% ~~\Arg{false code}
+% \end{syntax}
+% This function compares the full expansion of the \meta{test string}
+% in turn with the full expansion of the \meta{string cases}. If the two
+% full expansions are equal (as described for \cs{str_if_eq:nnTF} then the
+% associated \meta{code} is left in the input stream. If any of the
+% cases are matched, the \meta{true code} is also inserted into the
+% input stream (after the code for the appropriate case), while if none
+% match then the \meta{false code} is inserted. The function
+% \cs{str_case_x:nn}, which does nothing if there is no match, is also
+% available.
+% The \meta{test string} is expanded in each comparison, and must
+% always yield the same result: for example, random numbers must
+% not be used within this string.
+% \end{function}
+%
+% \section{Viewing strings}
+%
+% \begin{function}{\str_show:N, \str_show:c, \str_show:n}
+% \begin{syntax}
+% \cs{str_show:N} \meta{tl~var}
+% \end{syntax}
+% Displays the content of the \meta{str~var} on the terminal.
+% \end{function}
+%
+% \section{Scratch strings}
+%
+% \begin{variable}{\l_tmpa_str, \l_tmpb_str}
+% Scratch strings for local assignment. These are never used by
+% the kernel code, and so are safe for use with any \LaTeX3-defined
+% function. However, they may be overwritten by other non-kernel
+% code and so should only be used for short-term storage.
+% \end{variable}
+%
+% \begin{variable}{\g_tmpa_str, \g_tmpb_str}
+% Scratch strings for global assignment. These are never used by
+% the kernel code, and so are safe for use with any \LaTeX3-defined
+% function. However, they may be overwritten by other non-kernel
+% code and so should only be used for short-term storage.
+% \end{variable}
+%
+% \section{Internal \pkg{l3str} functions}
+%
+% \begin{function}[EXP]{\__str_to_other:n}
+% \begin{syntax}
+% \cs{__str_to_other:n} \Arg{token list}
+% \end{syntax}
+% Converts the \meta{token list} to a \meta{other string}, where
+% spaces have category code \enquote{other}. This function can be
+% \texttt{f}-expanded without fear of losing a leading space, since
+% spaces do not have category code $10$ in its result. It takes a
+% time quadratic in the character count of the string, but there exist
+% non-expandable ways to reach linear time.
+% \end{function}
+%
+% \begin{function}[EXP]{\__str_count_unsafe:n}
+% \begin{syntax}
+% \cs{__str_count_unsafe:n} \Arg{other string}
+% \end{syntax}
+% This function expects an argument that is entirely made of
+% characters with category \enquote{other}, as produced by
+% \cs{__str_to_other:n}. It leaves in the input stream the number of
+% character tokens in the \meta{other string}, faster than the
+% analogous \cs{str_count:n} function.
+% \end{function}
+%
+% \begin{function}[EXP]{\__str_range_unsafe:nnn}
+% \begin{syntax}
+% \cs{__str_range_unsafe:nnn} \Arg{other string} \Arg{start index} \Arg{end index}
+% \end{syntax}
+% Identical to \cs{str_range:nnn} except that the first argument is
+% expected to be entirely made of characters with category
+% \enquote{other}, as produced by \cs{__str_to_other:n}, and the
+% result is also an \meta{other string}.
+% \end{function}
+%
+% \section{Possible additions to \pkg{l3str}}
+%
+% Semantically correct copies of some \texttt{tl} functions.
+% \begin{itemize}
+% \item \cs{c_space_str}
+% \item \cs{str_clear:N}, \cs{str_gclear:N}, \cs{str_clear_new:N},
+% \cs{str_gclear_new:N}.
+% \item \cs{str_concat:NNN}, \cs{str_gconcat:NNN}
+% \item \cs{str_set_eq:NN}, \cs{str_gset_eq:NN}
+% \item \cs{str_if_empty:NTF}, \cs{str_if_empty_p:N}
+% \item \cs{str_if_exist:NTF}, \cs{str_if_exist_p:N}
+% \item \cs{str_use:N}
+% \end{itemize}
+%
+% Some functions that are not copies of \texttt{tl} functions.
+% \begin{itemize}
+% \item \cs{str_if_blank:NTF}, \cs{str_if_blank_p:N}.
+% \item \cs{str_map_inline:Nn}, \cs{str_map_function:NN},
+% \cs{str_map_variable:NNn}, and \texttt{:n} analogs.
+% \item Expandable \cs{str_if_in:nnTF}?
+% \item \cs{str_if_head_eq:nNTF}, \cs{str_if_head_eq_p:nN}
+% \item \cs{str_if_numeric/decimal/integer:n}, perhaps in \pkg{l3fp}?
+% \end{itemize}
+%
+% \end{documentation}
+%
+% \begin{implementation}
+%
+% \section{\pkg{l3str} implementation}
+%
+% \begin{macrocode}
+%<*initex|package>
+% \end{macrocode}
+%
+% \begin{macrocode}
+%<@@=str>
+% \end{macrocode}
+%
+% \begin{macrocode}
+\ProvidesExplPackage
+ {\ExplFileName}{\ExplFileDate}{\ExplFileVersion}{\ExplFileDescription}
+% \end{macrocode}
+%
+% The following string-related functions are currently defined in
+% \pkg{l3kernel}.
+% \begin{itemize}
+% \item \cs{str_if_eq:nn}[pTF] and variants,
+% \item \cs{str_if_eq_x_return:on}, \cs{str_if_eq_x_return:nn}
+% \item \cs{tl_to_str:n}, \cs{tl_to_str:N}, \cs{tl_to_str:c},
+% \item \cs{token_to_str:N}, \cs{cs_to_str:N}
+% \item \cs{str_head:n}, \cs{__str_head:w}, (copied here)
+% \item \cs{str_tail:n}, \cs{__str_tail:w}, (copied here)
+% \item \cs{__str_count_ignore_spaces} (unchanged)
+% \item \cs{__str_count_loop:NNNNNNNNN} (unchanged)
+% \end{itemize}
+%
+% \subsection{String assignments}
+%
+% \begin{macro}{\str_new:N, \str_new:c}
+% A string is simply a token list.
+% \begin{macrocode}
+\cs_new_eq:NN \str_new:N \tl_new:N
+\cs_generate_variant:Nn \str_new:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}
+% {
+% \str_set:Nn, \str_set:Nx,
+% \str_set:cn, \str_set:cx,
+% \str_gset:Nn, \str_gset:Nx,
+% \str_gset:cn, \str_gset:cx,
+% \str_const:Nn, \str_const:Nx,
+% \str_const:cn, \str_const:cx,
+% \str_put_left:Nn, \str_put_left:Nx,
+% \str_put_left:cn, \str_put_left:cx,
+% \str_gput_left:Nn, \str_gput_left:Nx,
+% \str_gput_left:cn, \str_gput_left:cx,
+% \str_put_right:Nn, \str_put_right:Nx,
+% \str_put_right:cn, \str_put_right:cx,
+% \str_gput_right:Nn, \str_gput_right:Nx,
+% \str_gput_right:cn, \str_gput_right:cx,
+% }
+% Simply convert the token list inputs to \meta{strings}.
+% \begin{macrocode}
+\tl_map_inline:nn
+ {
+ { set }
+ { gset }
+ { const }
+ { put_left }
+ { gput_left }
+ { put_right }
+ { gput_right }
+ }
+ {
+ \cs_new_protected:cpx { str_ #1 :Nn } ##1##2
+ { \exp_not:c { tl_ #1 :Nx } ##1 { \exp_not:N \tl_to_str:n {##2} } }
+ \exp_args:Nc \cs_generate_variant:Nn { str_ #1 :Nn } { Nx , cn , cx }
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{String variables and constants}
+%
+% \begin{variable}
+% {
+% \c_backslash_str,
+% \c_left_brace_str,
+% \c_right_brace_str,
+% \c_hash_str,
+% \c_tilde_str,
+% \c_percent_str
+% }
+% For all of those strings, use \cs{cs_to_str:N} to get characters with
+% the correct category code.
+% \begin{macrocode}
+\str_const:Nx \c_backslash_str { \cs_to_str:N \\ }
+\str_const:Nx \c_left_brace_str { \cs_to_str:N \{ }
+\str_const:Nx \c_right_brace_str { \cs_to_str:N \} }
+\str_const:Nx \c_hash_str { \cs_to_str:N \# }
+\str_const:Nx \c_tilde_str { \cs_to_str:N \~ }
+\str_const:Nx \c_percent_str { \cs_to_str:N \% }
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{variable}{\l_tmpa_str, \l_tmpb_str, \g_tmpa_str, \g_tmpb_str}
+% Scratch strings.
+% \begin{macrocode}
+\str_new:N \l_tmpa_str
+\str_new:N \l_tmpb_str
+\str_new:N \g_tmpa_str
+\str_new:N \g_tmpb_str
+% \end{macrocode}
+% \end{variable}
+%
+% \subsection{Counting characters}
+%
+% \begin{macro}[EXP]{\str_count_spaces:N, \str_count_spaces:n}
+% \begin{macro}[EXP, aux]{\@@_count_spaces_loop:wwwwwwwww}
+% To speed up this function, we grab and discard $9$ space-delimited
+% arguments in each iteration of the loop. The loop stops when the
+% last argument is one of the trailing |X|\meta{number}, and that
+% \meta{number} is added to the sum of $9$ that precedes, to adjust
+% the result.
+% \begin{macrocode}
+\cs_new_nopar:Npn \str_count_spaces:N
+ { \exp_args:No \str_count_spaces:n }
+\cs_new:Npn \str_count_spaces:n #1
+ {
+ \int_eval:n
+ {
+ \exp_after:wN \@@_count_spaces_loop:wwwwwwwww
+ \tl_to_str:n {#1} ~
+ X 7 ~ X 6 ~ X 5 ~ X 4 ~ X 3 ~ X 2 ~ X 1 ~ X 0 ~ X -1 ~
+ \q_stop
+ }
+ }
+\cs_new:Npn \@@_count_spaces_loop:wwwwwwwww #1~#2~#3~#4~#5~#6~#7~#8~#9~
+ {
+ \if_meaning:w X #9
+ \use_i_delimit_by_q_stop:nw
+ \fi:
+ \c_nine + \@@_count_spaces_loop:wwwwwwwww
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\str_count:N, \str_count:n, \str_count_ignore_spaces:n}
+% \begin{macro}[EXP, int]{\@@_count_unsafe:n}
+% \begin{macro}[EXP, aux]{\@@_count:n, \@@_count_loop:NNNNNNNNN}
+% To count characters in a string we could first escape all spaces
+% using \cs{@@_to_other:n}, then pass the result to \cs{tl_count:n}.
+% However, the escaping step would be quadratic in the number of
+% characters in the string, and we can do better. Namely, sum the
+% number of spaces (\cs{str_count_spaces:n}) and the result of
+% \cs{tl_count:n}, which ignores spaces. Since strings tend to be
+% longer than token lists, we use specialized functions to count
+% characters ignoring spaces. Namely, loop, grabbing $9$ non-space
+% characters at each step, and end as soon as we reach one of the $9$
+% trailing items. The \texttt{_unsafe} variant expects a token list
+% already converted to category code $12$ characters, and is used by
+% \cs{str_item:nn} and \cs{str_range:nnn}.
+% \begin{macrocode}
+\cs_new_nopar:Npn \str_count:N { \exp_args:No \str_count:n }
+\cs_new:Npn \str_count:n #1
+ {
+ \@@_count:n
+ {
+ \str_count_spaces:n {#1}
+ + \exp_after:wN \@@_count_loop:NNNNNNNNN \tl_to_str:n {#1}
+ }
+ }
+\cs_new:Npn \@@_count_unsafe:n #1
+ {
+ \@@_count:n
+ { \@@_count_loop:NNNNNNNNN #1 }
+ }
+\cs_new:Npn \str_count_ignore_spaces:n #1
+ {
+ \@@_count:n
+ { \exp_after:wN \@@_count_loop:NNNNNNNNN \tl_to_str:n {#1} }
+ }
+\cs_new:Npn \@@_count:n #1
+ {
+ \int_eval:n
+ {
+ #1
+ { X \c_eight } { X \c_seven } { X \c_six }
+ { X \c_five } { X \c_four } { X \c_three }
+ { X \c_two } { X \c_one } { X \c_zero }
+ \q_stop
+ }
+ }
+\cs_set:Npn \@@_count_loop:NNNNNNNNN #1#2#3#4#5#6#7#8#9
+ {
+ \if_meaning:w X #9
+ \exp_after:wN \use_none_delimit_by_q_stop:w
+ \fi:
+ \c_nine + \@@_count_loop:NNNNNNNNN
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Head and tail of a string}
+%
+% \begin{macro}[EXP]{\str_head:N, \str_head:n, \str_head_ignore_spaces:n}
+% \begin{macro}[EXP, aux]{\@@_head:w}
+% The \texttt{_ignore_spaces} variant is almost identical to
+% \cs{tl_head:n}. As usual, \cs{str_head:N} expands its argument and
+% hands it to \cs{str_head:n}. To circumvent the fact that \TeX{}
+% skips spaces when grabbing undelimited macro parameters,
+% \cs{@@_head:w} takes an argument delimited by a space. If |#1|
+% starts with a non-space character, \cs{use_i_delimit_by_q_stop:nw}
+% leaves that in the input stream. On the other hand, if |#1| starts
+% with a space, the \cs{@@_head:w} takes an empty argument, and the
+% single (initially braced) space in the definition of \cs{@@_head:w}
+% makes its way to the output. Finally, for an empty argument, the
+% (braced) empty brace group in the definition of \cs{str_head:n}
+% gives an empty result after passing through
+% \cs{use_i_delimit_by_q_stop:nw}.
+% \begin{macrocode}
+\cs_new_nopar:Npn \str_head:N { \exp_args:No \str_head:n }
+\cs_set:Npn \str_head:n #1
+ {
+ \exp_after:wN \@@_head:w
+ \tl_to_str:n {#1}
+ { { } } ~ \q_stop
+ }
+\cs_set:Npn \@@_head:w #1 ~ %
+ { \use_i_delimit_by_q_stop:nw #1 { ~ } }
+\cs_new:Npn \str_head_ignore_spaces:n #1
+ {
+ \exp_after:wN \use_i_delimit_by_q_stop:nw
+ \tl_to_str:n {#1} { } \q_stop
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\str_tail:N, \str_tail:n, \str_tail_ignore_spaces:n}
+% \begin{macro}[EXP, aux]{\@@_tail_auxi:w, \@@_tail_auxii:w}
+% Getting the tail is a little bit more convoluted than the head of a
+% string. We hit the front of the string with \cs{reverse_if:N}
+% \cs{if_charcode:w} \cs{scan_stop:}. This removes the first
+% character, and necessarily makes the test true, since the character
+% cannot match \cs{scan_stop:}. The auxiliary function then inserts
+% the required \cs{fi:} to close the conditional, and leaves the tail
+% of the string in the input stream. The details are such that an
+% empty string has an empty tail (this requires in particular that the
+% end-marker |X| be unexpandable and not a control sequence). The
+% \texttt{_ignore_spaces} is rather simpler: after converting the
+% input to a string, \cs{@@_tail_auxii:w} removes one undelimited
+% argument and leaves everything else until an end-marker \cs{q_mark}.
+% One can check that an empty (or blank) string yields an empty
+% tail.
+% \begin{macrocode}
+\cs_new_nopar:Npn \str_tail:N { \exp_args:No \str_tail:n }
+\cs_set:Npn \str_tail:n #1
+ {
+ \exp_after:wN \@@_tail_auxi:w
+ \reverse_if:N \if_charcode:w
+ \scan_stop: \tl_to_str:n {#1} X X \q_stop
+ }
+\cs_set:Npn \@@_tail_auxi:w #1 X #2 \q_stop { \fi: #1 }
+\cs_new:Npn \str_tail_ignore_spaces:n #1
+ {
+ \exp_after:wN \@@_tail_auxii:w
+ \tl_to_str:n {#1} \q_mark \q_mark \q_stop
+ }
+\cs_new:Npn \@@_tail_auxii:w #1 #2 \q_mark #3 \q_stop { #2 }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{Accessing specific characters in a string}
+%
+% \begin{macro}[EXP, int]{\@@_to_other:n}
+% \begin{macro}[EXP, aux]{\@@_to_other_loop:w, \@@_to_other_end:w}
+% First apply \cs{tl_to_str:n}, then replace all spaces by
+% \enquote{other} spaces, $8$ at a time, storing the converted part of
+% the string between the \cs{q_mark} and \cs{q_stop} markers. The end
+% is detected when \cs{@@_to_other_loop:w} finds one of the trailing
+% |A|, distinguished from any contents of the initial token list by
+% their category. Then \cs{@@_to_other_end:w} is called, and finds
+% the result between \cs{q_mark} and the first |A| (well, there is
+% also the need to remove a space).
+% \begin{macrocode}
+\group_begin:
+\char_set_lccode:nn { `\* } { `\ }
+\char_set_lccode:nn { `\A } { `\A }
+\tl_to_lowercase:n
+ {
+ \group_end:
+ \cs_new:Npn \@@_to_other:n #1
+ {
+ \exp_after:wN \@@_to_other_loop:w \tl_to_str:n {#1} ~ %
+ A ~ A ~ A ~ A ~ A ~ A ~ A ~ A ~ \q_mark \q_stop
+ }
+ \cs_new:Npn \@@_to_other_loop:w
+ #1 ~ #2 ~ #3 ~ #4 ~ #5 ~ #6 ~ #7 ~ #8 ~ #9 \q_stop
+ {
+ \if_meaning:w A #8
+ \@@_to_other_end:w
+ \fi:
+ \@@_to_other_loop:w
+ #9 #1 * #2 * #3 * #4 * #5 * #6 * #7 * #8 * \q_stop
+ }
+ \cs_new:Npn \@@_to_other_end:w \fi: #1 \q_mark #2 * A #3 \q_stop
+ { \fi: #2 }
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP, aux]{\@@_skip_c_zero:w}
+% \begin{macro}[EXP, aux]
+% {\@@_skip_loop:wNNNNNNNN, \@@_skip_end:w, \@@_skip_end:NNNNNNNN}
+% Removes |max(#1,0)| characters from the input stream, and then
+% leaves \cs{c_zero}. This should be expanded using
+% \cs{tex_romannumeral:D}. We remove characters $8$ at a time until
+% there are at most $8$ to remove. Then we do a dirty trick: the
+% \cs{if_case:w} construction leaves between $0$ and $8$ times the
+% \cs{or:} control sequence, and those \cs{or:} become arguments of
+% \cs{@@_skip_end:NNNNNNNN}. If the number of characters to remove
+% is $6$, say, then there are two \cs{or:} left, and the $8$ arguments
+% of \cs{@@_skip_end:NNNNNNNN} are the two \cs{or:}, and $6$
+% characters from the input stream, exactly what we wanted to
+% remove. Then close the \cs{if_case:w} conditional with \cs{fi:}, and
+% stop the initial expansion with \cs{c_zero} (see places where
+% \cs{@@_skip_c_zero:w} is called).
+% \begin{macrocode}
+\cs_new:Npn \@@_skip_c_zero:w #1;
+ {
+ \if_int_compare:w #1 > \c_eight
+ \exp_after:wN \@@_skip_loop:wNNNNNNNN
+ \else:
+ \exp_after:wN \@@_skip_end:w
+ \int_use:N \__int_eval:w
+ \fi:
+ #1 ;
+ }
+\cs_new:Npn \@@_skip_loop:wNNNNNNNN #1; #2#3#4#5#6#7#8#9
+ { \exp_after:wN \@@_skip_c_zero:w \int_use:N \__int_eval:w #1 - \c_eight ; }
+\cs_new:Npn \@@_skip_end:w #1 ;
+ {
+ \exp_after:wN \@@_skip_end:NNNNNNNN
+ \if_case:w #1 \exp_stop_f: \or: \or: \or: \or: \or: \or: \or: \or:
+ }
+\cs_new:Npn \@@_skip_end:NNNNNNNN #1#2#3#4#5#6#7#8 { \fi: \c_zero }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP, aux]{\@@_collect_delimit_by_q_stop:w}
+% \begin{macro}[EXP, aux]
+% {
+% \@@_collect_loop:wn, \@@_collect_loop:wnNNNNNNN,
+% \@@_collect_end:wn, \@@_collect_end:nnnnnnnnw
+% }
+% Collects |max(#1,0)| characters, and removes everything else until
+% \cs{q_stop}. This is somewhat similar to \cs{@@_skip_c_zero:w}, but
+% accepts integer expression arguments. This time we can only grab
+% $7$ characters at a time. At the end, we use an \cs{if_case:w}
+% trick again, so that the $8$ first arguments of
+% \cs{@@_collect_end:nnnnnnnnw} are some \cs{or:}, followed by an
+% \cs{fi:}, followed by |#1| characters from the input stream. Simply
+% leaving this in the input stream will close the conditional properly
+% and the \cs{or:} disappear.
+% \begin{macrocode}
+\cs_new:Npn \@@_collect_delimit_by_q_stop:w #1;
+ { \@@_collect_loop:wn #1 ; { } }
+\cs_new:Npn \@@_collect_loop:wn #1 ;
+ {
+ \if_int_compare:w #1 > \c_seven
+ \exp_after:wN \@@_collect_loop:wnNNNNNNN
+ \else:
+ \exp_after:wN \@@_collect_end:wn
+ \fi:
+ #1 ;
+ }
+\cs_new:Npn \@@_collect_loop:wnNNNNNNN #1; #2 #3#4#5#6#7#8#9
+ {
+ \exp_after:wN \@@_collect_loop:wn
+ \int_use:N \__int_eval:w #1 - \c_seven ;
+ { #2 #3#4#5#6#7#8#9 }
+ }
+\cs_new:Npn \@@_collect_end:wn #1 ;
+ {
+ \exp_after:wN \@@_collect_end:nnnnnnnnw
+ \if_case:w \if_int_compare:w #1 > \c_zero #1 \else: 0 \fi: \exp_stop_f:
+ \or: \or: \or: \or: \or: \or: \fi:
+ }
+\cs_new:Npn \@@_collect_end:nnnnnnnnw #1#2#3#4#5#6#7#8 #9 \q_stop
+ { #1#2#3#4#5#6#7#8 }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP]{\str_item:Nn, \str_item:nn, \str_item_ignore_spaces:nn}
+% \begin{macro}[EXP, aux]{\@@_item_unsafe:nn, \@@_item:ww}
+% The \cs{str_item:nn} hands its argument with spaces escaped to
+% \cs{@@_item_unsafe:nn}, and makes sure to turn the result back into
+% a proper string (with category code~$10$ spaces) eventually. The
+% \cs{str_item_ignore_spaces:nn} function cheats a little bit in that
+% it doesn't hand to \cs{@@_item_unsafe:nn} an \enquote{other string}.
+% This is safe, as everything else is done with undelimited arguments.
+% Then evaluate the \meta{index} argument~|#2| and count characters in
+% the string, passing those two numbers to \cs{@@_item:ww} for further
+% analysis. If the \meta{index} is negative, shift by |#2|, and
+% remove that number of characters before returning the next item in
+% the input stream (and if |#1| is smaller than $-|#2|$, nothing is
+% returned). If the \meta{index} is positive, ignore that number
+% (minus one) of characters before returning the next one. The shift
+% by one is obtained by inserting an empty brace group before the
+% string in that case: that brace group also covers the case where the
+% \meta{index} is zero.
+% \begin{macrocode}
+\cs_new_nopar:Npn \str_item:Nn { \exp_args:No \str_item:nn }
+\cs_new:Npn \str_item:nn #1#2
+ {
+ \exp_args:Nf \tl_to_str:n
+ {
+ \exp_args:Nf \@@_item_unsafe:nn
+ { \@@_to_other:n {#1} } {#2}
+ }
+ }
+\cs_new:Npn \str_item_ignore_spaces:nn #1
+ { \exp_args:No \@@_item_unsafe:nn { \tl_to_str:n {#1} } }
+\cs_new:Npn \@@_item_unsafe:nn #1#2
+ {
+ \exp_after:wN \@@_item:ww
+ \int_use:N \__int_eval:w #2 \exp_after:wN ;
+ \__int_value:w \@@_count_unsafe:n {#1} ;
+ #1 \q_stop
+ }
+\cs_new:Npn \@@_item:ww #1; #2;
+ {
+ \int_compare:nNnTF {#1} < \c_zero
+ {
+ \int_compare:nNnTF {#1} < {-#2}
+ { \use_none_delimit_by_q_stop:w }
+ {
+ \exp_after:wN \use_i_delimit_by_q_stop:nw
+ \tex_romannumeral:D \exp_after:wN \@@_skip_c_zero:w
+ \int_use:N \__int_eval:w #1 + #2 ;
+ }
+ }
+ {
+ \int_compare:nNnTF {#1} > {#2}
+ { \use_none_delimit_by_q_stop:w }
+ {
+ \exp_after:wN \use_i_delimit_by_q_stop:nw
+ \tex_romannumeral:D \@@_skip_c_zero:w #1 ; { }
+ }
+ }
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP, aux]{\@@_range_normalize:nn}
+% This function converts an \meta{index} argument into an explicit
+% position in the string (a result of $0$ denoting \enquote{out of
+% bounds}). Expects two explicit integer arguments: the
+% \meta{index} |#1| and the string count~|#2|. If |#1| is negative,
+% replace it by $|#1| + |#2| + 1$, then limit to the range $[0,
+% |#2|]$.
+% \begin{macrocode}
+\cs_new:Npn \@@_range_normalize:nn #1#2
+ {
+ \int_eval:n
+ {
+ \if_int_compare:w #1 < \c_zero
+ \if_int_compare:w #1 < -#2 \exp_stop_f:
+ \c_zero
+ \else:
+ #1 + #2 + \c_one
+ \fi:
+ \else:
+ \if_int_compare:w #1 < #2 \exp_stop_f:
+ #1
+ \else:
+ #2
+ \fi:
+ \fi:
+ }
+ }
+% \end{macrocode}
+% \end{macro}
+%
+% \begin{macro}[EXP]
+% {\str_range:Nnn, \str_range:nnn, \str_range_ignore_spaces:nnn}
+% \begin{macro}[EXP, int]{\@@_range_unsafe:nnn}
+% \begin{macro}[EXP, aux]{\@@_range:www, \@@_range:nnw}
+% Sanitize the string. Then evaluate the arguments. At this stage we
+% also decrement the \meta{start index}, since our goal is to know how
+% many characters should be removed. Then limit the range to be
+% non-negative and at most the length of the string (this avoids
+% needing to check for the end of the string when grabbing
+% characters), shifting negative numbers by the appropriate amount.
+% Afterwards, skip characters, then keep some more, and finally drop
+% the end of the string.
+% \begin{macrocode}
+\cs_new_nopar:Npn \str_range:Nnn { \exp_args:No \str_range:nnn }
+\cs_new:Npn \str_range:nnn #1#2#3
+ {
+ \exp_args:Nf \tl_to_str:n
+ {
+ \exp_args:Nf \@@_range_unsafe:nnn
+ { \@@_to_other:n {#1} } {#2} {#3}
+ }
+ }
+\cs_new:Npn \str_range_ignore_spaces:nnn #1
+ { \exp_args:No \@@_range_unsafe:nnn { \tl_to_str:n {#1} } }
+\cs_new:Npn \@@_range_unsafe:nnn #1#2#3
+ {
+ \exp_after:wN \@@_range:www
+ \__int_value:w \@@_count_unsafe:n {#1} \exp_after:wN ;
+ \int_use:N \__int_eval:w #2 - \c_one \exp_after:wN ;
+ \int_use:N \__int_eval:w #3 ;
+ #1 \q_stop
+ }
+\cs_new:Npn \@@_range:www #1; #2; #3;
+ {
+ \exp_args:Nf \@@_range:nnw
+ { \@@_range_normalize:nn {#2} {#1} }
+ { \@@_range_normalize:nn {#3} {#1} }
+ }
+\cs_new:Npn \@@_range:nnw #1#2
+ {
+ \exp_after:wN \@@_collect_delimit_by_q_stop:w
+ \int_use:N \__int_eval:w #2 - #1 \exp_after:wN ;
+ \tex_romannumeral:D \@@_skip_c_zero:w #1 ;
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+% \end{macro}
+%
+% \subsection{String conditionals}
+%
+% \begin{macro}[EXP, pTF]{\str_if_eq:NN}
+% \begin{macro}[EXP, pTF]{\str_if_eq:nn, \str_if_eq_x:nn}
+% Note that \cs{str_if_eq:NN} is different from
+% \cs{tl_if_eq:NN} because it needs to ignore category codes.
+% \begin{macrocode}
+\prg_new_conditional:Npnn \str_if_eq:NN #1#2 { p , TF , T , F }
+ {
+ \if_int_compare:w \__str_if_eq_x:nn { \tl_to_str:N #1 } { \tl_to_str:N #2 }
+ = \c_zero \prg_return_true: \else: \prg_return_false: \fi:
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% \begin{macro}[EXP, TF]{\str_case:nn, \str_case:on, \str_case_x:nn}
+% Defined in \pkg{l3basics} at present.
+% \end{macro}
+%
+% \subsection{Viewing strings}
+%
+% \begin{macro}{\str_show:n, \str_show:N, \str_show:c}
+% Displays a string on the terminal.
+% \begin{macrocode}
+\cs_new_eq:NN \str_show:n \tl_show:n
+\cs_new_eq:NN \str_show:N \tl_show:N
+\cs_generate_variant:Nn \str_show:N { c }
+% \end{macrocode}
+% \end{macro}
+%
+% \subsection{Deprecated string functions}
+%
+% Deprecated 2013-01-20 for removal by 2013-04-30
+% \begin{macro}[EXP]
+% {\str_substr:Nnn, \str_substr:nnn, \str_substr_ignore_spaces:nnn}
+% \begin{macro}[EXP, aux]{\@@_substr:nnn}
+% These functions used to allow for an empty argument to denote the
+% start/end of the string. We reimplement them here by first checking
+% for an empty argument, then only calling the appropriate version of
+% the \cs{str_range:nnn} function.
+% \begin{macrocode}
+\cs_new:Npn \str_substr:Nnn #1 { \@@_substr:nnn { \str_range:Nnn #1 } }
+\cs_new:Npn \str_substr:nnn #1 { \@@_substr:nnn { \str_range:nnn {#1} } }
+\cs_new:Npn \str_substr_ignore_spaces:nnn #1
+ { \@@_substr:nnn { \str_range_ignore_spaces:nnn {#1} } }
+\cs_new:Npn \@@_substr:nnn #1#2#3
+ {
+ \tl_if_empty:nTF {#2}
+ { \tl_if_empty:nTF {#3} { #1 { 1} { -1 } } { #1 { 1} {#3} } }
+ { \tl_if_empty:nTF {#3} { #1 {#2} { -1 } } { #1 {#2} {#3} } }
+ }
+% \end{macrocode}
+% \end{macro}
+% \end{macro}
+%
+% Deprecated 2013-01-20 for removal by 2013-04-30
+% \begin{variable}{\c_lbrace_str, \c_rbrace_str}
+% \begin{macrocode}
+\cs_new_eq:NN \c_lbrace_str \c_left_brace_str
+\cs_new_eq:NN \c_rbrace_str \c_right_brace_str
+% \end{macrocode}
+% \end{variable}
+%
+% \begin{macrocode}
+%</initex|package>
+% \end{macrocode}
+%
+% \end{implementation}
+%
+% \PrintIndex