Hi All,

I am new to shell script. And right now learning regular expression. I am using BASH as a shell and RedHat Linux. I have couple of doubts regarding the regular expression

1) +
This is matches one or more occurrence of the preceding pattern.
So if the file contains following data
file.txt
a
aa
aba
bbb
ccc

Code:

grep -i 'a+' file.txt

should print following
a
aa
aba

But to print above I need to have

Code:

 grep -i a\+' file.txt

2) ?
should match 0 or 1 occurrence of the preceding pattern

But when I use

Code:

 grep -i 'a?' file.txt

it pritnts nothing.. And when I use

Code:

grep -i 'a\?' file.txt

it pritints all the data in the file. i.e output was as below
a
aa
aba
bbb
ccc

I was expecting the output as below
a
bbb
ccc

Now I have a doubt 0 or 1 means exactly 0 or 1 ?? or is it mean 0 or more then 0 ??? if it means 0 or more then 0 then whats the differnece between * (asterisk ) and ? ( question mark )??


3)

Code:

grep -i 'a\{1,\}' file.txt

and

Code:

grep -i 'a\{1\}' file.txt

prints the same data as below

a
aa
aba

I was expecting this output only when I use

Code:

grep -i 'a\{1,\}' file.txt

and should print below line when I use

Code:

grep -i 'a\{1\}' file.txt

a

Please Please help me.........
Thnx in advance