Question

I have a database unload file with field separated with the <TAB> character. I am running this file through sed to replace any occurences of <TAB><TAB> with <TAB>\N<TAB>. This is so that when the file is loaded into MySQL the \N in interpreted as NULL.

The sed command ‘s/\t\t/\t\N\t/g;’ almost works except that it only replaces the first instance e.g. “…<TAB><TAB><TAB>…” becomes “…<TAB>\N<TAB><TAB>…”.

If I use ‘s/\t\t/\t\N\t/g;s/\t\t/\t\N\t/g;’ it replaces more instances.

I have a notion that despite the /g modifier this is something to do with the end of one match being the start of another.

Could anyone explain what is happening and suggest a sed command that would work or do I need to loop.

I know I could probably switch to awk, perl, python but I want to know what is happening in sed.

Answer 1

I know you want sed, but sed doesn’t like this at all, it seems that it specifically (see here) won’t do what you want. However, perl will do it (AFAIK):

perl -pe 'while (s#\t\t#\t\n\t#) {}' <filename>