About Log Merge

I want to merge logs based on this format [2024-03-09 17:03:02] to observe stack information,I configured it this way,But I found that this configuration is not effective, and it seems that promtail does not recognize merging starting with [, I tried adding [ before ,I tried using 2.8.2 and 2.9.2
firstline: "'^[\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}

Not clear on what you are trying to achieve. Can you provide an example?