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# 1  
Old 05-17-2009
grep regular expression
please can someone tell me what the following regrex means

grep "^aa*$" <file>

I thought this would match any word beginning with aa and ending with $, but it doesnt.

Thanks in advance

Calypso
# 2  
Old 05-17-2009
Quote:
I thought this would match any word beginning with aa
You are right!

The command filter contains only lines which start with "aa"!
# 3  
Old 05-17-2009
Quote:
The command filter contains only lines which start with "aa"!
this
Code:
echo "aaaa" | grep "^aa*$"

returns aaaa

but this
Code:
echo "aaa1" | grep "^aa*$"

returns nothing

Calypso
# 4  
Old 05-17-2009
Quote:
Originally Posted by research3
...The command filter contains only lines which start with "aa"!
Nope, the "grep" regex matches all the lines in <file> that have one or more a's and nothing other than a's.

Code:
$
$ cat -n input.txt
     1  a
     2  aa
     3  aaa
     4  aba
     5  abb
     6
     7  b
     8  A
     9  AA
$
$ grep -n "^aa*$" input.txt
1:a
2:aa
3:aaa
$
$

tyler_durden
# 5  
Old 05-17-2009
Quote:
Originally Posted by Calypso
...but this
Code:
echo "aaa1" | grep "^aa*$"

returns nothing...
The second command returns nothing because there is a character other than a in the input stream.

The grep regex will match only a's. And there must be at least one a.

tyler_durden
# 6  
Old 05-17-2009
Thanks for that,

I just had one more question about grep, is it possible to match possible multiple patterns for example how could i say find lines that match

1. "^aa.*$" AND ends with "aa"

2. "^aa.*$" OR ends with "aa"

Thanks
Calypso
# 7  
Old 05-18-2009
Quote:
Originally Posted by Calypso
1. "^aa.*$" AND ends with "aa"
Code:
grep "^a*aa$" <file>

Quote:
Originally Posted by Calypso
2. "^aa.*$" OR ends with "aa"
Code:
egrep "^aa*$|aa$" <file>

tyler_durden
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