Suppose that you have several databases with names 'e25.fdb', 'e26.fdb', 'e27.fdb', ...
If you want to trace activity in any of them it is reasonable do not enumerate all these names in the trace config; rather specify just pattern.

1) For 2.5 trace config will be like this:
<database (%[\\/](e[[:DIGIT:]]{2}).fdb)>
    enabled true
     . . .
</database>

It is enough to launch ISQL and check whether we use correct pattern in <database> section:

echo set list on; select 1 from rdb$database where 'C:\MIX\firebird\QA\fbt-repo\tmp\e25.fdb' similar to '%[\\/](e[[:DIGIT:]]{2}).fdb'; | C:\MIX\firebird\fb25\bin\isql /:employee

Output on 2.5.9.27115 will be: 1, i.e. all fine.

2) For 3.0+ we have to modify 'database' section and replace "< ... >" with "{... }" because of changed syntax, but the pattern will (and must) be the same:

database = (%[\\/](e[[:DIGIT:]]{2}).fdb)
{
  enabled = true
   . . .
}

But this does not work.
Trace log will be like this:
===
Trace session ID 1 started
Error creating trace session for database "":
Passed text: illegal line <database = (%[\\/](e[[:DIGIT:]]{2}).fdb)>
===

AFAIU, the problem relates to "{" and "}" that are used within pattern.
If we change 'database' section in such manner that no curvy brackets will be used than all again fine:

database = (%[\\/](e[[:DIGIT:]][[:DIGIT:]]).fdb) # this DOES work, no errors.
{
  . . .
}


Checked on: WI-T4.0.0.1198 ; WI-V3.0.4.33045.