Allow forward slashes in regexp

Because of the way the filter detects a regexp, it does not allow using forward slashes in the pattern. This is quite inconvenient when trying to filter messages matching URLs paths.

The regexp has been adjusted to accept escaped forward slashes, for example:
/\\/users\\/\\d+/

Two tests that fail in the current version have been added:
- A regexp with the right pattern must be created created when using escaped forward slashes.
- The right messages are emitted when using a rule defined by a regexp including escaped forward slashes.