Custom pattern in grok debugger

Hi I'm trying to set up a grok filter for custom logs, look like this:
2023-01-20 00:00:05.235+0000 [L: DEBUG] [O: S.c.t.d.e.DSLScript] [I: ] [U: Administrator] [S: ] [P: ] [T: TWEventProcessor-3] @@@ Property Write Queue: 0

And I'm stuck at the stage of describing the [L: DEBUG] piece. I have a regex, that works fine:
^\[[A-Z]{1}(: ){1}[A-Z]{4,10}\]$
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But when I'm trying to create a custom pattern in specified field in grok debugger and simulate it, I get a simulate error:
image

This is how all the information I enter into the grok debugger looks like:
Sample data:
2023-01-20 00:00:05.235+0000 [L: DEBUG]
Grok pattern:
%{TIMESTAMP_ISO8601:timestamp} %{TWX_LOGLEVEL:loglevel}
Custom patterns:
TWX_LOGLEVEL ^\[[A-Z]{1}(: ){1}[A-Z]{4,10}\]$

Thanks in advance

I found the mistake.
TWX_LOGLEVEL \[[A-Z]{1}(: ){1}[A-Z]{4,10}\]
I had to delete symbols of beginning and ending of regex