Automatically replace all |foo| with \abs{foo}

Question

Technically speaking, this isn't a LaTeX specific question, but there are so many useful tools in LaTeX, and I'm using it to solve a LaTeX problem, so I thought here would be a good place to ask.

I have a rather large document with lots of math notation. I've used |foo| throughout to indicate the absolute value of foo. I'd like to replace every instance of |foo| with \abs{foo}, so that I can control the notation via an abs macro I define.

Does anybody know of a way to do this? Possibly using regular expressions of some sort? Are there editors with tools built in for this?

Answer 1

Option 1: sed

The stream editing tool, sed, would be a natural first choice, but the problem is that sed can't match non-greedy regular expressions.

We need a non-greedy regular expression here- to clarify why, let's consider

sed -r 's/|(.*)|/\\abs{\1}/g' myfile.tex

If we apply this substitution to a file that contains something like

$|a|+|b|\geq|a+b|$

then we'll get

$\abs{a|+|b|\geq|a+b}$

which is clearly not what we want- regular expression matches like this are greedy by default.

To make it non-greedy, we typically use .*?, but the problem is that sed does not support this type of match. Happily (thanks Hendrik) we can use the following instead

sed -r 's/\|([^|]*)\|/\\abs{\1}/g' myfile.tex

Once you're comfortable that it does what you want, you can use

sed -ri.bak 's/\|([^|]*)\|/\\abs{\1}/g' myfile.tex

which will overwrite each file, and make a back up first, myfile.tex.bak

Option 2: perl

We could, instead, use a little perl one-liner:

perl -pe 's/\|(.*?)\|/\\abs{\1}/g' myfile.tex

When you're sure that you trust it is working correctly, you can use the following to overwrite myfile.tex

perl -pi -e 's/\|(.*?)\|/\\abs{\1}/g' myfile.tex

You can replace myfile.tex with, for example, *.tex to operate on all the .tex files in the current working directory.

Details of perl's switches are discussed here (for example): http://perldoc.perl.org/perlrun.html#Command-Switches

Answer 2

\documentclass{article}

\begin{document}

\def\abs#1{|#1|} % Let us assume that it is your definition

$\abs{c+d}$

\catcode`\|=\active
\def|#1|{\abs{#1}}

$|a|+|b|\geq|a+b|$

\end{document}

We assume that |'s are in pairs. Then the first one becomes definition, the second -- end of an argument. It seems, however, that, as for all the other solutions, a manual correction will be necessary.

Answer 3

You can also try WinEdt. It has a very well implemented 'search and replace' feature.

In your case, search for (with regex on)

\|\(0*\)\|

and replace with

\\abs\{\0\}

The whole replacement can be done in one step.

You can also choose (with a different regex) to make the replacement ONLY inside math environments.

Answer 4

You can use Vim to achieve this. Please also have a look here.

In your case, the command would be

%s:\(|\)\(.\{-}\)\(|\):\\abs{\2}:gc

Brief explanation:

%s - says it is the substitute command over the entire file

\( | \) matches the occurrence of | and references it as \1 in the match

\( .\{-} \) matches anything between two instances of | but makes it as small as possible, it references this chunk as \2 which we later use to put it back

\( | \) again matches another occurrence of | and references it as \3 in the match

: signals end of regex

\\abs{\2} is what you want to replace it with, where \2 is the text between the two |

:gc says everywhere in the line, and muse be confirmed.

You must confirm every replacement because this string is not perfect. Reason? If there is an odd number of | in a line then it cannot tell which two instances of | form the text in between that you are taking the absolute value of. However, it will not skip over lines, so you are relatively protected.

Answer 5

This post is inspired by cmhughes proposed solutions. His post is one of the most interesting posts on TeX editing which I have ever read. I just spent 2 hours trying to produce nawk solution which actually doesn't exist (see below).

Option 3: Gawk

This solution is provided by Hendrik Vogt

gawk '{print gensub(/\|([^|]*)\|/, "\\abs{\\1}", "g", $0)}'

Option 4: Python

import sys
import re

file = sys.stdin.readlines()
for line in file:
    newline = re.sub('\|(.*?)\|', '\\\\abs{\\1}', line)
    sys.stdout.write(newline)

Why is AWK (nawk) the wrong tool for the above problem?

AWK doesn't support non-greedy regular expressions which is to be expected since it is sed's cousin but even worse AWK regular expression does not capture its groups. Even if AWK was supporting group capturing we would be in trouble as backreferences cannot be used inside character classes and we use character classes to achieve non-greediness in AWK.

A simple AWK script

NR>0{
gsub(/\|([^|]*)\|/,"\\abs{\1}")
print
}

Applied to file

$|abs|$ so on and so fourth
$$|a|+|b|\geq|a+b|$$
who is affraid of wolf $|abs|$

will unfortunately produce

$\abs{}$ so on and so fourth
$$\abs{}+\abs{}\geq\abs{}$$
who is affraid of wolf $\abs{}$

Answer 6

On Mac OSX I use TextWrangler, it is an editor which—among its many features—has a Search/Replace facility able to handle regular expressions. For example I can choose:

The first time you click “Next” and then you can keep clicking “Replace & Find” to go through your document, validating each match before a replacement is made. This will lower the potential of messing up your document because of using an incorrect regular expression or making unintended replacements. And with infinite “Undo” you'll always be safe.