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Passing regular expression to nawk

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# 1  
Old 07-17-2013
Passing regular expression to nawk
I am trying to test if I can replace a regular expression in 'nawk' with a variable.
Please let me know why it is not working. I am using ksh88i on solaris8
I am trying use this test as a building block to filter active external DNS connections.

Ideally I want to pass variable defined outside nawk as an argument to it.

Code:
# Here I am printing only DNS names.
$> echo server1.domain1.domain2.com  | nawk '/^[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*/{print}'
server1.domain1.domain2.com 

# Here I am trying to pass same pattern as a variable, its not working.
$>echo server1.domain1.domain2.com | nawk -v re='^[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*' '/re/{print}'

Last edited by kchinnam; 07-17-2013 at 01:45 AM.. Reason: typo
# 2  
Old 07-17-2013
below might work with some warnings..

Code:
 
tools>  echo server1.domain1.domain2.com | awk -v "re=^[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*" '$0 ~ re{print}'
awk: warning: escape sequence `\.' treated as plain `.'
server1.domain1.domain2.com

# 3  
Old 07-17-2013
Slight modification still:
Code:
echo "server1.domain1.domain2.com" | awk -v re='^[a-z0-9]*\\.[a-z0-9]*\\.[a-z0-9]*\\.[a-z0-9]*$' '$0~re{print}''

--
@viyadhar: the back slashes need to be escaped within single quotes otherwise the regex in the string will match anything (the dots will not be interpreted as literal dots).
@kchinnam: you probably need a $-sign at the end as well?
Last edited by Scrutinizer; 07-17-2013 at 02:19 AM..
# 4  
Old 07-17-2013
My question is really using 'nawk' I am able to get the first command working, but not the second one with parameter, what am I doing wrong?

Code:
$> echo server1.domain1.domain2.com | nawk '/^[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*/ {print}'
server1.domain1.domain2.com

$>echo server1.domain1.domain2.com | nawk -v re='^[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*\.[a-z0-9]*' '/re/ {print}'
<nothing gets printed>

# 5  
Old 07-17-2013
Within awk you can use a so-called regex constant / regex /{print} or $0 ~ / regex / {print } but a regex constant cannot contain a variable.

If you want to use a variable you need to put the regex in a string and use the regex matching operator ~ ( $0 ~ " regex "{print} )

So it becomes -v re='regex' ' .... $0 ~ re {print }. But this means the backslashes do need to be escaped, because the string needs to be evaluated first before it can be interpreted as a regex, hence the double backslashes in my previous example ( in a string you need to use \\ is you want to print \ ).
Last edited by Scrutinizer; 07-17-2013 at 06:37 PM..
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