matches Does the protocol or text string match the given Perl regular expression?
fix.MsgType == "D" and fix.SecurityDesc matches "CLT|NGT"
The "matches" operator allows a filter to apply to a specified Perl-compatible regular expression (PCRE). The "matches" operator is only implemented for protocols and for protocol fields with a text string representation.
More information on PCRE can be found in the pcrepattern(3) man page (Perl Regular Expressions are explained in http://perldoc.perl.org/perlre.html).