I am trying to do a assignment for university, but I am currently stuck.
The goal is to read some phone numbers and reverse the order of the first 3 digits and put them in brackets. I can get it to read the phone numbers but not do the reversal of the digits.

ex: input

214 4234-5555

ex: output

412 4234-5555

this is what I have so far

sed -r "s/([0-9]), ([0-9]), ([0-9])/321/g" phone.txt

To modify OP's attempt

$ cat ip.txt
214 4234-5555
foo 123 4533-3242

$ sed -r 's/([0-9])([0-9])([0-9])/321/' ip.txt
412 4234-5555
foo 321 4533-3242

$ # adding parenthesis as well
$ sed -r 's/([0-9])([0-9])([0-9])/(321)/' ip.txt
(412) 4234-5555
foo (321) 4533-3242

$ # if ERE is not supported
$ sed 's/([0-9])([0-9])([0-9])/(321)/' ip.txt
(412) 4234-5555
foo (321) 4533-3242

• Note that some sed implementation would need -E instead of -r
  


• Use single quotes unless you need interpolation, see also https://mywiki.wooledge.org/Quotes
  


• 
  ([0-9]), ([0-9]), ([0-9]) means matching 3 digits separated by comma and space


• 
  g modifier is needed if all matches in a line should be changed

For a generic solution, i.e defining number of digits to reverse as a numeric argument

$ perl -pe 's/d3/reverse $&/e' ip.txt
412 4234-5555
foo 321 4533-3242
$ perl -pe 's/d3/sprintf "(%s)", scalar reverse $&/e' ip.txt
(412) 4234-5555
foo (321) 4533-3242

Here's a very long, convoluted, and probably unnecessary sed, but here it is nonetheless because fun:

sed -re 'h; s/^([0-9]*) *(.*)/1n/; :1 s/(.)(.*n)/21/;t1; s/.//; s/^(.*)$/(1)/; x;s/([0-9]3)(.*)/2/;x;G;s/n//'

This works as so:

 # pretend 214 4234-5555 is the current line
h; # copy the current line into hold space
s/^([0-9]*) *(.*)/1n/; # keep only first 3 numbers, 214
:1 s/(.)(.*n)/21/;t1; s/.//; # reversing string in sed, 
 # see notes below; 214 becomes 412
s/^(.*)$/(1)/; # After string is reversed, add brackets; (412)
x;s/([0-9]3)(.*)/2/; # swap hold and pattern buffer, 
 # delete first 3 chars; 
 # pattern space now is <space>4234-5555

x;G;s/n// # swap again, append hold buffer to pattern buffer; 
 # now pattern buffer is (412)<newline> 4234-5555; 
 # finally delete newline; we get (412) 4234-5555

And that's how it looks like in action:

$ printf "214 4234-5555n123 3333n" | sed -re 'h; s/^([0-9]*) *(.*)/1n/; :1 s/(.)(.*n)/21/;t1; s/.//; s/^(.*)$/(1)/; x;s/([0-9]3)(.*)/2/;x;G;s/n//'
(412) 4234-5555
(321) 3333

Note: String reversal originally found on Stephane Chazelas's comment

$ n='214 4234-5555'
$ echo `echo $n:0:3|rev`$n:3:9
412 4234-555

Method1

I have used below method get the same result

i=`awk 'print $1' example.txt| rev`
awk -v i="$i" 'print i,$2' example.txt

Output

412 4234-5555

Method2

sed 's/(.)(.)(.)/321/' example.txt

output

412 4234-5555

If you have the number in phone.txt as "xxx xxx-xxxx", then you can use the below:

!/bin/bash

echo '('$(cat phone.txt | cut -d ' ' -f1 | rev)')'