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This is a follow-up to a previous question I had asked. For that question, I received an excellent answer by @markp-fuso demonstrating the use of an array of search patters for sed to perform a particular substitution task.

In this question, I would like to perform the same find and replace task of replacing all forms of pow(var,2) with square(var) but I would like to do this by using a regex variable.

The sample input and output files are below:

InputFile.txt:

pow(alpha,2) + pow(beta,2)
(3*pow(betaR_red,2))
2/pow(gammaBlue,3))
-pow(epsilon_gamma,2)+5

OutputFile.txt:

square(alpha) + square(beta)
(3*square(betaR_red))
2/pow(gammaBlue,3))
-square(epsilon_gamma)+5

After looking at this answer and some more tinkering on https://regex101.com/r/80Hp5Z/1/, I was able to describe the text between pow( and ,2) using (?<=pow\().*?(?=,2\)).

Using Bash, I am trying to form a two command solution where:

  1. First command sets var to (?<=pow\().*?(?=,2\))
  2. Second command performs sed "s/pow(${var},2)/square(${var})/g" InputFile.txt > OutputFile.txt

It seems once I figure out how to set var successfully in Step 1, I can proceed with the command in Step 2. However, after reading this question and this question I tried the following two commands but they did not perform any substitutions:

bash$ var="(?<=pow\().*?(?=,2\))"
bash$ sed "s/pow(${var},2)/square(${var})/g" InputFile.txt > OutputFile.txt

I would really appreciate some help forming a 2-command Bash solution as described above that makes use of a regex variable and transforms InputFile.txt into OutputFile.txt.

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    Probably a much better solution overall is to write your program in such a way that you can switch between these functions without changing the code, either via a #define (which obviously requires you to recompile the program, but is very easy to manage from a Makefile or similar) or even by rewriting the program so you can switch at run-time (via a command-line option then, or perhaps an environment variable). Commented Oct 3, 2020 at 20:15
  • @tripleee: Thanks, this idea is very interesting and I didn't know you can do this. If possible, could you please give me some tips or references on how I can write such a #define statement that allows seamless switching between pow(var,2) and square(var)? Commented Oct 3, 2020 at 20:25
  • How to replace pow(foo(),2) would have been a far more interesting question since matching one ) is trivial but matching an unknown number of nested (...)s isn't. Commented Oct 3, 2020 at 21:02
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    Create a simple wrapper function which accepts one argument var, then returns either pow(var, 2) or square(var) depending on your #define. Post a new question if you can't figure it out (though I would guess it has been asked before; search before asking). Commented Oct 4, 2020 at 9:14
  • Ah, I see what you are trying to say. This is really cool and I can see myself using this in other situations as well. Thanks @tripleee. Commented Oct 4, 2020 at 19:13

2 Answers 2

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If you want to deal with parentheses in squared expressions, try:

One-line Solution:

perl -pe 's/pow\s*\((([^()]|\((?2)+\))+)\s*,\s*2\s*\)/square($1)/g' inputFile.txt

Two-Line Solution:

REGEX="pow\s*\((([^()]|\((?2)+\))+)\s*,\s*2\s*\)"
perl -pe "s/$REGEX/square(\1)/g" inputFile.txt

https://regex101.com/r/Dp0Duf/1.

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5 Comments

Alexander Mashin, thanks a lot for the perl solution, it works perfectly. Would it be possible to make this into a two-command solution, where the first command sets var = a regex and then in the second command, this regex variable gets used in the perl command?
@procyon, create a file called, say, t: #!/bin/bash REGEX="pow\s*\((([^()]|\((?2)+\))+)\s*,\s*2\s*\)" perl -pe "s/$REGEX/square($1)/g" $1 Make it executable and call it like this: ./t file. P.S. The line breaks should be before REGEX and perl.
Thanks, I tried out your bash script using ./t file, but it converts pow(alpha,2) to square(file) instead of square(alpha). Could you please tell me why this happens when it looks equivalent to your one-liner after variable expansion?
Oh, sorry, it's funny. Two $1's are mixed up. Try this: #!/bin/bash REGEX="pow\s*\((([^()]|\((?2)+\))+)\s*,\s*2\s*\)" perl -pe "s/$REGEX/square(\1)/g" $1
Thanks so much, Alexander Mashin. I've accepted your answer and added your updated bash script as a two-line bash solution to your answer as a suggested edit. I sincerely appreciate your help.
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You may use this sed with one capture group:

s='pow(\([^)]+),2\)'
sed -E "s/$s/square\1)/g" file

square(alpha) + square(beta)
(3*square(betaR_red))
2/pow(gammaBlue,3))
-square(epsilon_gamma)+5

RegEx Demo

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5 Comments

Thanks, I tried this and it works! I'm trying to use (?<=pow\().*?(?=,2\)) (regex101.com/r/80Hp5Z/1). Would it be possible to use that in a sed substitution command to get the same result?
No, you cannot use lookahead and lookbehind in sed.
Thanks @anubhava. One of the things I am confused about in my question is how to first set the regex part of a sed command as a variable and then use it later on in the actual sed command. Would it be possible to add a two-liner solution to your answer where the first command sets var to a regex and in the second command var gets used by sed to perform the substitution?
Sorry I went to sleep after previous comment. I have updated my answer.
Your two step solution will work really well with a workflow involving 1) tinkering on regex101.com and setting a regex variable 2) applying the sed command. Thanks @anubhava, I appreciate your help!

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