Regular Expression Trouble

Paul A. Procacci pprocacci at
Wed Aug 27 03:26:32 UTC 2008

Martin McCormick wrote:
> I am trying to isolate only the MAC addresses that appear in
> dhcpd logs.
> For anyone who is interested, the sed construct that should do
> this looks like:
>  sed 's/.*\([[ your regular expression ]]\).*/\1/' 
> The \1 tells sed to only print what matched and skip all the rest.
> 	I am doing something wrong with the regular expression
> that is supposed to recognise a MAC address. MAC addresses look
> like 5 pairs of hex digits followed by :'s and then a 6TH pair
> to end the string.
> 	I have tried:
> [[:xdigit:][:xdigit:][:punct:]
> Sorry. It won't all fit on a line, but there should be a string
> of 5 pairs and the : and then the 6TH pair followed by the
> closing ] so the expression ends with ]]
> One should also be able to put:
> [[:xdigit:][:xdigit:][:punct:]]\{5,5\}[[:xdigit:][:xdigit]]
> Any ideas as to what else I can try?
> What happens is I get single characters per line that look like
> the first or maybe the last character in that line, but
> certainly nothing useful or nothing that remotely looks like a
> MAC address.
> 	Any ideas as to what's wrong with the regular
> expression?
> 	Many thanks.
> Martin McCormick WB5AGZ  Stillwater, OK 
> Systems Engineer
> OSU Information Technology Department Telecommunications Services Group
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I don't have a seperate dhcp log and you didn't make it clear if you do, 
but I do have something similar written for awk that parses the system 
log file.

awk ' /DHCPREQUEST/ { print $10 } '  /var/log/messages

Maybe that will help.


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