Macro for removing whitespace from the start of a string

Question

I want a macro that somehow replaces whitespace (spaces, newlines, tabs?) from the start of a string.

Something like

mystring.replace(/^\s*/g, "")

in JavaScript.

Is that possible?

Answer 1

Expandable, infinite spaces

Removing space tokens at the begin of a string is possible, but a little cumbersome:

\documentclass{article}

\makeatletter
\let\TrSp@X\relax
\let\TrSp@Y\relax
\edef\TrSp@XSpXNil{%
  \TrSp@X\space\TrSp@X
  \noexpand\noexpand\noexpand\TrSp@NIL
}
\long\edef\trim@pre@spaces#1{%
  \noexpand\romannumeral0%
  \noexpand\TrSp@pre@spaces\TrSp@X#1\TrSp@Y\TrSp@XSpXNil
}
\@firstofone{\long\def\TrSp@pre@spaces#1\TrSp@X} %
#2\TrSp@X#3\TrSp@NIL{%
  \csname TrSp@pre#3\endcsname{#1}{#2}%
}
\long\def\TrSp@pre#1#2{%
  \expandafter
  \TrSp@pre@\expandafter\space\@gobble#1%
}
\long\def\TrSp@pre@#1\TrSp@Y{#1}
\long\expandafter\edef\csname TrSp@pre X\endcsname#1#2{%
  \noexpand\TrSp@pre@spaces\TrSp@X#2\TrSp@XSpXNil
}
\def\TrSp@X{X}
\let\TrSp@Y\TrSp@NIL
\let\trimstartspaces\trim@pre@spaces
\newcommand*{\trimstartspacesin}[1]{%
  \expandafter\expandafter\expandafter\expandafter
  \expandafter\expandafter\expandafter\def
  \expandafter\expandafter\expandafter\expandafter
  \expandafter\expandafter\expandafter#1%
  \expandafter\expandafter\expandafter\expandafter
  \expandafter\expandafter\expandafter{%
    \expandafter\trimstartspaces\expandafter{\foo}%
  }%
}
\makeatother


% create a text with several spaces in a row using a trick:
\newcommand*{\deffoo}[1]{%
  \newcommand*{\foo}{#1#1#1#1a b c }%
}
\deffoo{ }
\let\trimmedfoo\foo
\trimstartspacesin\trimmedfoo

\typeout{[\expandafter\trimstartspaces\expandafter{\foo}]}
\typeout{[\trimmedfoo]}

\begin{document}
[\foo] $\rightarrow$ [\expandafter\trimstartspaces\expandafter{\foo}]
  or [\trimmedfoo]
\end{document}

Expandable, at most one space

Package trimspaces already provides \trim@pre@space or \trim@pre@space@in.

Manually it can be done by

\romannumeral-`\.

\romannumeral applied to negative numbers vanishes and TeX keeps looking for an optional space after the character constant `\..

If the text is inside a macro \foo, then an optional space can be stripped at the beginning by:

\expandafter\def\expandafter\foo\expandafter{%
  \romannumeral-\expandafter`\expandafter\.\foo
}%

Full example:

\documentclass{article}
\newcommand*{\foo}{ a b c }
\expandafter\def\expandafter\trimmedfoo\expandafter{%
  \romannumeral-\expandafter`\expandafter\.\foo
}%
\typeout{[\meaning\trimmedfoo]}

\begin{document}
[\foo] $\rightarrow$ [\trimmedfoo]
\end{document}

Usually removing one optional space is enough, because it is not too easy entering more than one space in a row (TeX collapses them to one space during tokenizing).

Non-expandable, infinite spaces

\@ifnextchar removes spaces before testing the "next" character. In non expandable contexts it can be used to remove infinite spaces from the begin of the text:

\documentclass{article}

% create a text with several spaces in a row using a trick:
\newcommand*{\deffoo}[1]{%
  \newcommand*{\foo}{#1#1#1#1a b c }%
}
\deffoo{ }

\makeatletter
\newcommand*{\stripstartspaces}{%
  \expandafter\@ifnextchar\expandafter X\expandafter{\expandafter
  }\expandafter{\expandafter}%
}
\makeatother

\begin{document}
[\foo] $\rightarrow$ [\stripstartspaces\foo]
\end{document}

Answer 2

The stringstrings package has a \removeleadingspaces macro, which will remove any combination of blanks and hard spaces at the the beginning of the passed text string. The package is generally intended to operate only upon text strings, and has only a very limited ability (not shown in this answer) to handle arguments containing macros.

By default, the macro will output the result immediately to the screen. However, if the processed but unprinted result is desired, a [q] optional argument will instead put the result in \thestring via an \edef for later access. There is also an [e] "encoded" mode, which applies if one is trying to process strings containing a limited number of user-predefined macros (but that's for a different question).

\documentclass{article}
\usepackage{stringstrings}
\begin{document}
.\removeleadingspaces{abc def}

.\removeleadingspaces{ abc def}

.\removeleadingspaces{  abc def}

.\removeleadingspaces{~abc def}

.\removeleadingspaces{~~abc def}

.\removeleadingspaces{~ ~ abc def}

.\removeleadingspaces{~ ~ abc def}
\end{document}