Question about using grep

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127.0.0.1 - - [08/Mar/2011:00:26:35 +0530] "HEAD /sk/ HTTP/1.1" 200 169 "-" "Python-urllib/2.6"
127.0.0.1 - - [08/Mar/2011:00:26:35 +0530] "HEAD /sk/.treeinfo HTTP/1.1" 404 182 "-" "Python-urllib/2.6"
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127.0.0.1 - - [08/Mar/2011:00:26:35 +0530] "HEAD /sk/Client HTTP/1.1" 404 182 "-" "Python-urllib/2.6"
127.0.0.1 - - [08/Mar/2011:00:26:35 +0530] "HEAD /sk/RedHat HTTP/1.1" 404 182 "-" "Python-urllib/2.6"
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I am having a file of above structure and it is name log.txt. Now I do

 cat log.txt | grep '[*]' 

there was no output then I did

 cat log.txt | grep '\[* \]' 

again there was no output but when I did

 cat log.txt | grep '\[.*\]'

then there was output how is this working?

UPTADE

I am basically trying to look for the time stamps in above log.

cat log.txt | grep '[*]' 

This command line searches the file log.txt for lines that contain a * in them. Since none of the lines in your input have * you get nothing. Note that [..] in regex is called a character class which matches any of the character inside it.

cat log.txt | grep '\[* \]' 

This command line searches the file for lines that have zero or more of literal [ followed by a space and followed by a literal ]. Again you don't have any such lines. Here you've escaped the [ and ]. This makes them non-special. So they don't form a character class anymore but are treated literally.

cat log.txt | grep '\[.*\]'

This one searches for lines that have a [ followed by anything (.*) followed by a ]. Since you have such lines you get the output.

Finally

 cat file | grep pattern

is called useless use of cat and can be written as:

grep pattern file

grep uses a regular expression as a pattern. The one you wrote (last one) means

• [ : the '[' character once
• .* : any character (.) any times from 0 to infinity (*)
• ] : the ']' character once

Writing \[*\] would mean "The '[' character any number of times and then the ']' character". As writing [*] wouldn't mean anything since [] is the marker for a group of characters. There is a short explanation of regular expressions in man grep and you may find much more online

Please see Regular Expression Basic Syntax Reference

In regexes the point "." means "any character" and the star "*" means: match me if the character before me is present 0 or more times.
"[" and "]" are used to limit a class of characters.

So:

1. "[*]" matches only a star
2. "\[*\[" means take any '[' followed by a ']'
3. "\[.*\]" take anything which starts with a '[' followed by any char, followed by a '\]'