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This is a really simple question, but even after reading about wildcards/regex, I can't seem to grasp entirely what the differences between these commands are:

grep . test.txt

this one will look for lines containing any character in test.txt

grep \. test.txt

this one also apparently does the exact same thing, even though I assumed it shouldn't, since it would be escaped and return only those lines containing an actual '.'

grep ? test.txt

since ? is a special character, I guess it would be interpreted as a wildcard, but since grep doesn't deal with wildcards, it's not going to output anything; however

grep \? test.txt

does find the lines which contain an '?' in them, but also those that contain an '\?' in them. Why is this?

jasonwryan
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Kocka Mijalkova
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    In grep \., the shell sees the backslash as an escape character, but a dot doesn't need escaping, so it's converted to .. So both commands look the same to grep. Try grep '\.'. – Mikel May 24 '15 at 19:17
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    For ? there are several things to point out. First, grep gets to see the unescaped ? because you don't have one-character filenames in the current directory. Second, \? is expanded by the shell to ?. And third, ? has no special meaning for grep. It starts being special for egrep, or for grep -E, or grep -P. Plain grep expects basic regexps (BRE), and ? is nor special in the BRE dialect. – lcd047 May 24 '15 at 19:29

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