Replacing first occurence in every line

Question

Supposing I have a document formatted like this:

word1 word2 word3
word4 word5 word6
word7 word8

How do I use sed to replace the f

Answer 1

sed 's/\([^[:blank:]]\{1,\}\)/\1,/' YourFile

Add a , after first block of "non space", so start of line could be starting with space, it still work and also if line is without work but only with space (no , in this case)