Why is sed not recognizing \t as a tab?

Question

sed \"s/\\(.*\\)/\\t\\1/\" $filename > $sedTmpFile && mv $sedTmpFile $filename

I am expecting this sed script to insert a <

Answer 1

Instead of BSD sed, i use perl:

ct@MBA45:~$ python -c "print('\t\t\thi')" |perl -0777pe "s/\t/ /g"
   hi