[replace] How to replace multiple patterns at once with sed?

Suppose I have 'abbc' string and I want to replace:

• ab -> bc
• bc -> ab

If I try two replaces the result is not what I want:

echo 'abbc' | sed 's/ab/bc/g;s/bc/ab/g'
abab

So what sed command can I use to replace like below?

echo abbc | sed SED_COMMAND
bcab

EDIT: Actually the text could have more than 2 patterns and I don't know how many replaces I will need. Since there was a answer saying that sed is a stream editor and its replaces are greedily I think that I will need to use some script language for that.

I believe this should solve your problem. I may be missing a few edge cases, please comment if you notice one.

You need a way to exclude previous substitutions from future patterns, which really means making outputs distinguishable, as well as excluding these outputs from your searches, and finally making outputs indistinguishable again. This is very similar to the quoting/escaping process, so I'll draw from it.

• s/\\/\\\\/g escapes all existing backslashes
• s/ab/\\b\\c/g substitutes raw ab for escaped bc
• s/bc/\\a\\b/g substitutes raw bc for escaped ab
• s/\\\(.\)/\1/g substitutes all escaped X for raw X

I have not accounted for backslashes in ab or bc, but intuitively, I would escape the search and replace terms the same way - \ now matches \\, and substituted \\ will appear as \.

Until now I have been using backslashes as the escape character, but it's not necessarily the best choice. Almost any character should work, but be careful with the characters that need escaping in your environment, sed, etc. depending on how you intend to use the results.

sed is a stream editor. It searches and replaces greedily. The only way to do what you asked for is using an intermediate substitution pattern and changing it back in the end.

echo 'abcd' | sed -e 's/ab/xy/;s/cd/ab/;s/xy/cd/'

This might work for you (GNU sed):

sed -r '1{x;s/^/:abbc:bcab/;x};G;s/^/\n/;:a;/\n\n/{P;d};s/\n(ab|bc)(.*\n.*:(\1)([^:]*))/\4\n\2/;ta;s/\n(.)/\1\n/;ta' file

This uses a lookup table which is prepared and held in the hold space (HS) and then appended to each line. An unique marker (in this case \n) is prepended to the start of the line and used as a method to bump-along the search throughout the length of the line. Once the marker reaches the end of the line the process is finished and is printed out the lookup table and markers being discarded.

N.B. The lookup table is prepped at the very start and a second unique marker (in this case :) chosen so as not to clash with the substitution strings.

With some comments:

sed -r '
  # initialize hold with :abbc:bcab
  1 {
    x
    s/^/:abbc:bcab/
    x
  }

  G        # append hold to patt (after a \n)

  s/^/\n/  # prepend a \n

  :a

  /\n\n/ {
    P      # print patt up to first \n
    d      # delete patt & start next cycle
  }

  s/\n(ab|bc)(.*\n.*:(\1)([^:]*))/\4\n\2/
  ta       # goto a if sub occurred

  s/\n(.)/\1\n/  # move one char past the first \n
  ta       # goto a if sub occurred
'

The table works like this:

   **   **   replacement
:abbc:bcab
 **   **     pattern

Here is a variation on ooga's answer that works for multiple search and replace pairs without having to check how values might be reused:

sed -i '
s/\bAB\b/________BC________/g
s/\bBC\b/________CD________/g
s/________//g
' path_to_your_files/*.txt

Here is an example:

before:

some text AB some more text "BC" and more text.

after:

some text BC some more text "CD" and more text.

Note that \b denotes word boundaries, which is what prevents the ________ from interfering with the search (I'm using GNU sed 4.2.2 on Ubuntu). If you are not using a word boundary search, then this technique may not work.

Also note that this gives the same results as removing the s/________//g and appending && sed -i 's/________//g' path_to_your_files/*.txt to the end of the command, but doesn't require specifying the path twice.

A general variation on this would be to use \x0 or _\x0_ in place of ________ if you know that no nulls appear in your files, as jthill suggested.

Here is an awk based on oogas sed

echo 'abbc' | awk '{gsub(/ab/,"xy");gsub(/bc/,"ab");gsub(/xy/,"bc")}1'
bcab

I always use multiple statements with "-e"

$ sed -e 's:AND:\n&:g' -e 's:GROUP BY:\n&:g' -e 's:UNION:\n&:g' -e 's:FROM:\n&:g' file > readable.sql

This will append a '\n' before all AND's, GROUP BY's, UNION's and FROM's, whereas '&' means the matched string and '\n&' means you want to replace the matched string with an '\n' before the 'matched'

echo "C:\Users\San.Tan\My Folder\project1" | sed -e 's/C:\\/mnt\/c\//;s/\\/\//g'

replaces

C:\Users\San.Tan\My Folder\project1

to

mnt/c/Users/San.Tan/My Folder/project1

in case someone needs to replace windows paths to Windows Subsystem for Linux(WSL) paths

Tcl has a builtin for this

$ tclsh
% string map {ab bc bc ab} abbc
bcab

This works by walking the string a character at a time doing string comparisons starting at the current position.

In perl:

perl -E '
    sub string_map {
        my ($str, %map) = @_;
        my $i = 0;
        while ($i < length $str) {
          KEYS:
            for my $key (keys %map) {
                if (substr($str, $i, length $key) eq $key) {
                    substr($str, $i, length $key) = $map{$key};
                    $i += length($map{$key}) - 1;
                    last KEYS;
                }
            }
            $i++;
        }
        return $str;
    }
    say string_map("abbc", "ab"=>"bc", "bc"=>"ab");
'

bcab

If replacing the string by Variable, the solution doesn't work. The sed command need to be in double quotes instead on single quote.

#sed -e "s/#replacevarServiceName#/$varServiceName/g" -e "s/#replacevarImageTag#/$varImageTag/g" deployment.yaml

May be a simpler approach for single pattern occurrence you can try as below: echo 'abbc' | sed 's/ab/bc/;s/bc/ab/2'

My output:

 ~# echo 'abbc' | sed 's/ab/bc/;s/bc/ab/2'
 bcab

For multiple occurrences of pattern:

sed 's/\(ab\)\(bc\)/\2\1/g'

Example

~# cat try.txt
abbc abbc abbc
bcab abbc bcab
abbc abbc bcab

~# sed 's/\(ab\)\(bc\)/\2\1/g' try.txt
bcab bcab bcab
bcab bcab bcab
bcab bcab bcab

Hope this helps !!