Perl optional capture groups not working?

Question

I have the following sample.txt file:

2021-10-07 10:32:05,767 ERROR [LAWT2] blah.blah.blah - Message processing FAILED: <ExecutionReport blah="xxx" foo="yyy" SessionID="kkk" MoreStuff="zz"> Total time for which application threads were stopped: 0.0003858 seconds, Stopping threads took: 0.0000653 seconds
2021-10-07 10:31:32,902 ERROR [LAWT6] blah.blah.blah - Message processing FAILED: <NewOrderSingle SessionID="zkx" TargetSubID="ttt" Account="blah" MsgType="D" BookingTypeOverride="0" Symbol="6316" OtherField1="othervalue1" Otherfield2="othervalue2"/></D></NewOrderSingle>

I want to grab just two key fields: "SessionID" and "MsgType" and print like this:

SessionID="kkk"|
SessionID="zkx"|MsgType="D"

In other words: if the group match is not there, I want just to print blank.

I've tried the following approach but no luck:

$$ perl -ne '/ (SessionID=".*?")? .*(MsgType=".*?")? / and print "$1|$2\n"' sample.txt
SessionID="kkk"|
SessionID="zkx"|

Can somebody enlighten me here? Thank you a lot.

Answer 1

You can use

perl -ne '/\h(SessionID="[^"]*")?(?:\h++.*(MsgType="[^"]*"))?\h/ and print "$1|$2\n"' 

See the regex demo. Details:

• \h - a horizontal whitespace
• (SessionID="[^"]*")? - Group 1: an optional SessionID=", any zero or more chars other than ", and then a "
• (?:\h++.*(MsgType=".*?"))? - an optional (but greedy) sequence of
  • \h++ - one or more horizontal whitespaces
  • .* - any zero or more chars other than line break chars as many as possible
  • (MsgType="[^"]*") - Group 2: SessionID=", any zero or more chars other than ", and then a "
• \h - a horizontal whitespace.

See the online demo:

s='2021-10-07 10:32:05,767 ERROR [LAWT2] blah.blah.blah - Message processing FAILED: <ExecutionReport blah="xxx" foo="yyy" SessionID="kkk" MoreStuff="zz"> Total time for which application threads were stopped: 0.0003858 seconds, Stopping threads took: 0.0000653 seconds
2021-10-07 10:31:32,902 ERROR [LAWT6] blah.blah.blah - Message processing FAILED: <NewOrderSingle SessionID="zkx" TargetSubID="ttt" Account="blah" MsgType="D" BookingTypeOverride="0" Symbol="6316" OtherField1="othervalue1" Otherfield2="othervalue2"/></D></NewOrderSingle>'
perl -ne '/\h(SessionID=".*?")?(?:\h++.*(MsgType=".*?"))?\h/ and print "$1|$2\n"' <<< "$s"

This prints:

SessionID="kkk"|
SessionID="zkx"|MsgType="D"