I have a file in which I want to change all of the code that has the following format : n{,3}L{,2}n{,5} where n= [0-9] any number and L [a-zA-Z] any letter either capital or not

I want to change A or a into AB and d or D into DK, something like this:

 Annnnn--> ABnnnnn ; Dnnn-->DKnnn 

the file looks like:

$ cat filename

I tried the following sed script

sed 's/[aA]/AB/g;s/[dD]/DK/g' filename

it works for instances which have only A or D but for those which are already AB or DK it adds up the letter as AB--> ABB or DK-->DKK. Any help appreciated with explanation. Thanks!


As for what's wrong with your script, you are replacing A or a with AB and D or d with DK, so any pre-existing B or K would not be affected; sed is not looking for it. You could put an optional [bB] or [kK] using ? (zero or one of the preceding character) to make it replace that character too if it occurs.

To make sure the replacement only happens if [aA] or [aA][bB] etc is followed by a number, you can add the number to the pattern and add it back in the replacement with () and \1

sed -r 's/ab?([0-9])/AB\1/Ig;s/dk?([0-9])/DK\1/Ig' filename

I am using -r to use ERE (so no need to escape ?) and I for case-insensitive search instead of using character classes.

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  • @Daniel it's a pleasure :D – Zanna Dec 1 '16 at 22:21
  • a Question! I just realized that your script would not successful if there would be another letter already in the filename. for instance it will also change the nnAYnnn--> nnABYnnn which I do not want to see such change and want to keep this intact and observe only changes for code has only "A" or "AB". Do you have any idea about it? – Daniel Dec 1 '16 at 22:35
  • @Daniel ah so, if we have [Aa], change it to [AB] only if the next character after [Aa] is [0-9], or [bB] and then [0-9]? – Zanna Dec 1 '16 at 22:42
  • Yes, Exactly! :) – Daniel Dec 1 '16 at 22:45
  • @Daniel I've edited, I think I understood correctly, please comment if not :) – Zanna Dec 1 '16 at 22:52

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