Sed replace word except when word is preceded by a specific string

Question

I want to replace all instance of abc with 123 provided it isn't preceded by https://.

Code:

file=Data.txt
Initial="# Start";
orig="abc";
new="123"; 

Final="$" # Line Number
sed -r -e "/${Initial}/,${Final}s/${orig}/${new}/g" ${file} # Final doesn't accept $

Data:

# Start

abc.md
https://abc.md
The path is https://abc.md for abc.md
The path for abc.md is https://abc.md 

Expected Output:

# Start

123.md
https://abc.md
The path is https://abc.md for 123.md
The path for 123.md is https://abc.md 

How can I achieve this?

---

Note: I also need to use ${Initial} and ${Final} to denote between which the pattern might exist.

This answer doesn't work for the case The path is: https://abc.md abc.md

Answer 1

What you are looking for is negative-look-behind, which neither sed or awk supports. I recommend going with perl, e.g.:

file=Data.txt
export Initial="# Start"
export orig="abc"
export new="123"
export Final="5"

perl -pe '
 $flag=1 if /$ENV{Initial}/;
 s,(?<!https://)$ENV{orig},$ENV{new},g if $flag;
 $flag=0 if $. == $ENV{Final};
' $file

Answer 2

You could temporarily change all instances that start with https://to something else, not containing abc, and change them back when you've finished.

Lazy method

sed -e 's_https://abc_protected_g;/# Start/,$s_abc_123_g;s_protected_https://abc_'

You'll have to pick some value for the temporary string, that you can be certain won't appear in your input (and doesn't contain any special characters that would interfere with sed or shell quoting). I used protected but something more distinctive is advisable.

Thorough method

If you can't be certain that your temporary string won't occur in the input, a longer but safer alternative would be to use two replacement strings:

sed -e 's/X/Xv/g;s_https://abc_Xu_g;/^# Start/,$s/abc/123/g;s_Xu_https://abc_g;s/Xv/X/g'

The requirements of these two temporary strings are:

• They have a common prefix (I used X, but it can be longer/more readable)
• They don't occur within the other variables: orig, new, Initial or Final
• As before, they don't contain characters that would break the sed expression