How to create an "AppendToLineIfNotContains" rule

Hello,

I am in the process of migrating from cfe2 to cfe3 and am stuck with this 
problem:

I need to append a string to a line only...
...if this line exists AND
...if the string is not already in the line

Example: A firewall config file that contains IP aliases like so:

WEB_SERVERS=10.0.0.5,10.0.06,10.0.0.8,10.0.0.42

Now when a new web server is being added, its IP should be appended to the 
line:

,10.0.0.66

Cfengine2 had a command 
<https://docs.cfengine.com/docs/archive/manuals/cf2-Reference.html#AppendToLineIfNotContains> 
for that, it would look like this:

editfiles:
  any::
  { /etc/firewall/aliases.cfg
    LocateLineMatching "^WEB_SERVERS=.*"
    AppendToLineIfNotContains ",10.0.0.66"
  }

I am trying to replicate this behaviour. The only thing I could come up 
with was this:

bundle agent firewall
{
  files:
    any::
      "/etc/firewall/aliases.cfg"
      edit_line => add_webserver;
}

bundle edit_line add_webserver
{
  replace_patterns:
    "^WEB_SERVERS=(.*)"
      replace_with => appendToLine(",10.0.0.66"),
      if => not(regline("^WEB_SERVERS=.*,10\.0\.0\.66", 
"/etc/firewall/aliases.cfg"));
}

body replace_with appendToLine(text)
{
  replace_value => "$(match.0)$(text)"; # backreference
  occurrences => "first";          # first, last all
}

This does not work, it complains that the "regular expression matches the 
replacement string":

*error: Promised replacement 
'WEB_SERVERS=10.0.0.5,10.0.06,10.0.0.8,10.0.0.42,10.0.0.66' on line 
'WEB_SERVERS=10.0.0.5,10.0.06,10.0.0.8,10.0.0.42' for pattern 
'^WEB_SERVERS=(.*')' is not convergent while editing 
'/etc/firewall/aliases.cfg' (regular expression matches the replacement 
string)*

How would I fix this?

I know that there is field editing 
<https://docs.cfengine.com/docs/3.19/reference-promise-types-files-edit_line-field_edits.html> 
in cfe3, but we might need this in other places which don't necessarily all 
have a field structure.

Thanks,
Jay

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