Below is the codec which am using for multiline events.
codec => multiline {
pattern => "%{TIMESTAMP_ISO8601:syslogtime}\s%{WORD:str}\s%{WORD:s}\s%{YEAR:yeaa}-%{MONTHNUM:ooo}-%{MONTHDAY:ppp}\s%{TIME:trrrs}"
#pattern => "((^%{TIMESTAMP_ISO8601} %{WORD} %{WORD} %{YEAR}-%{MONTHNUM}-%{MONTHDAY} %{TIME}))"
negate => true
what => "previous"
}