my config for customEvents is

                                                {
                                                keyword: "Gerburtstag",
                                                symbol: "birthday-cake",
                                                transform: {           
                                                        search: "^([^\\s]+) (\\d{4}) Gerburtstag$",           
                                                        replace: "$1 ($2)",           
                                                        yearmatchgroup: 2
                                                        }
                                                } 

my test calendar event title (from the summary field in the actual ICS data) is

SUMMARY:test 1986 Gerburtstag

the debug line in the parser is

[2025-05-25 16:08:47.371] [DEBUG] title: test 1986 Gerburtstag 

I see

I walked thru the code and it worked as expected…

I got the feeling i am not far away from the solution. Have still patient to bring me to that point? :)

as I noted in my prior post
also remove the leading /

^(.+) (\\d{4}) Geburtstag$

its captures EVERYTHING from the start… and there is nothing left for the remainder of the expression

you need to capture any character UP to the 1st space,

^([^\s]+) (\d{4}) Gerburstag$

I use https://regex101.com/ to test (see below)

escape the backslash as required

Does help you?

module: "calendar",
header: "Familienkalender",
position: "top_left",
                   config:{
                          loglevel: ["DEBUG"],
                                        customEvents: [
                                                {
                                                keyword: "Geburtstag",
                                                symbol: "birthday-cake",
                                                transform: {
                                                        search: "^([^\']*) \'(\\d{4})$",
                                                        replace: "$1 ($2)",
                                                        yearmatchgroup: 2
                                                        }
                                                }

the transform happens in the front end, the backend (node_helper) gets the events from source and formats them for frontend.tye log you showed is from the backend

I got the message that there is one entry but no title is shown. Did I do something wrong?

,"DEBUG"

to the config.js logLevel property
as well as your transform
and run MM with (stop pm2 version , with pm2 stop MagicMirror)

npm start >somefile.txt 2>&1

so we can see the actual text of the event
(watch out, the log output contains the COMPLETE text of the calendar url)

I only need the text of the event… (it supposedly has the date in it)

i would use the developers window sources tab to locate that code in the calendar module and examine the data it is working with

i might get a chance later today

I tried to figure out. Went to the console tab and put the filter on cal.
I get an error your yearmatch is that right? But I don’t know how to deal with it. Can you give me a hint?

open the developers window
ctrl-shift-i
select the console tab
put cal in the filter field
and you may see the error , I changed to match one of my cal event titles
but the regex doesn’t work…

Next I tried to use the transform option:

                {
                        module: "calendar",
                        header: "Familienkalender",
                        position: "top_left",
                                config:{
                                        customEvents: [
                                                {
                                                keyword: "Geburtstag",
                                                symbol: "birthday-cake",
                                                transform: {           
                                                        search: "^(.+) (\\d{4}) Geburtstag$",           
                                                        replace: "$1 ($2)",           
                                                        yearmatchgroup: 2
                                                        }
                                                }       
                                                        ],
                                        calendars: [
                                                {
                                                url: "my-url“,
                                                symbolClass: "fas fa-fw fa-"
                                                }
                                                  ]
                                        }
                },

But when I refresh the browser whithin the Kalender „Familienkalender“ is no entry anymore but the word „loading…“ but nothing new is shown after a while.

What could this circumstance cause?

rather like this

           {
                        module: "calendar",
                        header: "Familienkalender",
                        position: "top_left",
                                config: {
                                        customEvents: [
                                                        {
                                                        keyword: "Geburtstag",
                                                        symbol: "fire"
                                                        }
                                                ],                                    
                                       calendars: [
                                               {
                                                     url: "my-url“,
                                                    symbolClass: "fas fa-fw fa-",
                                              }
                                       ]
                               }
                },

the calendar module documentation is organized into sections

1. applies to all calendar definitions (refresh rate, maximumDays…etc)
2. applies to A calendar definition (color, symbol, url)