Difference between "?" and "*"

Gaurang033 · #1 (**permalink**) 07-10-2008

Hi frnds,

Can anybody please tell me what is the difference between "?" and "*" Regular expression.

I have file called test.txt as follows
a
aa
aba
pqr
xyz

Both

Code:

egrep "a?" test.txt

and

Code:

egrep "a*" test.txt

both prints the same output. the Whole file.

fpmurphy · #2 (**permalink**) 07-10-2008

egrep uses EREs (Extended Regular Expressions) From POSIX.2

Quote:

An atom followed by `*' matches a sequence of 0 or more matches of the atom. An atom followed by `+' matches a sequence of 1 or more matches of the atom. An atom followed by `?' matches a sequence of 0 or 1 matches of the atom.

Thus "a?" implies 0 or 1 matches of "a" and "a*" implies 0 or more matches of "a".

If you want to just list the lines containing a, the following will work

Code:

$ egrep "a+" file
a
aa
aba
$

Gaurang033 · #3 (**permalink**) 07-10-2008

Quote:

Originally Posted by fpmurphy

Thus "a?" implies 0 or 1 matches of "a" and "a*" implies 0 or more matches of "a".

That's what i am asking. I know "a?" means 0 or 1 occurrence of preceding pattern. I was thinking it as exactly 0 or 1 occurrence. but it's not like that. I am not getting the difference between "*" and "?" . If there is any can you give me some example of it.

krishmaths · #4 (**permalink**) 07-10-2008

@gaurang:

Remember that you are doing a grep. That means the command will list down lines from the file that match the input pattern.

In this case egrep works on your patterns a? and a* in the following way.

egrep a? --> List down lines from test.txt if it contains an a followed by any character. Which means a, aa, abcdef, a897 all will match, and hence you get all these in the output.

egrep a* --> List down lines from test.txt if it contains an a followed by 0 or more nr of characters. Again a, aa, abcdef, a897 all will match.

Hope you get the point.

Moreover, to see the difference between ? and *, create files with names : a, aa, a897, abcd
and then do

Code:

ls a?

and

Code:

ls a*

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