Support escaped octal and hexadecimal characters in regular expressions

In the previous commit, we did not support characters in regular
expressions specified via \0..., \x... or \u... yet. This is a bit more
involved, therefore support for them is added in its own commit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a unit test for regular expressions

We do not really support regular expressions yet, but we do support
trivial patterns including ones with escaped characters. Let's make sure
that that works as advertised.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>