What does this PR do?

After several tests, it appears that when building containers using docker-compose with the documentation pattern ("\d{4}\-(0?[1-9]|1[012])\-(0?[1-9]|[12][0-9]|3[01])"), no logs whatsoever are coming in.
According to Nils:

the pattern is not recognised as valid then. And when we fail to parse the configuration we do not tail the container at all (hopefully this will get changed in 6.7 - not 6.6)

This was tested with Postgresql logs, that do not start with a timestamp:

• When using this "\d{4}\-(0?[1-9]|1[012])\-(0?[1-9]|[12][0-9]|3[01])" as regex pattern, no logs at all are coming in
• When using this "\\d{4}\-(0?[1-9]|1[012])\-(0?[1-9]|[12][0-9]|3[01])" as regex pattern, logs are coming in
• When using "foo" as regex pattern, logs are coming in

It was also tested with Nginx logs, that do start with a timestamp

• When using this "\d{4}\-(0?[1-9]|1[012])\-(0?[1-9]|[12][0-9]|3[01])" as regex pattern, no logs at all are coming in
• When using this "\\d{4}\-(0?[1-9]|1[012])\-(0?[1-9]|[12][0-9]|3[01])" as regex pattern, logs are coming in

I performed several other tests that seem to converge toward this same conclusion but I could definitely use a double check by someone else.
Customer ended up with this conclusion as well:

• First ticket: https://datadog.zendesk.com/agent/tickets/174263
• Second ticket: https://datadog.zendesk.com/agent/tickets/178123

More details about this on this Slack convo:
https://dd.slack.com/archives/C6QR6DXJ4/p1540287215000100
https://cl.ly/c48752cc034f

Motivation

Preview link

Additional Notes