Question

Unix question please:
What is the difference between
grep .... file.txt
and
grep -w .... file.txt
What role does the wildcard play here? Like how is .... interpreted when it is all dots and no letters are used?
Thank you.

Answer 1

-w, --word-regexp
Select only those lines containing matches that form whole words. The test is that the matching substring must either be at the beginning of the line, or preceded by a non-word constituent character. Similarly, it must be either at the end of the line or followed by a non-word constituent character. Word-constituent characters are letters, digits, and the underscore.

From the man page.