lighttpd2 uses the "Perl-compatible regular expressions" implementation from GLib, see their "Regular expression syntax":https://developer.gnome.org/glib/stable/glib-regex-syntax.html documentation.
The config format has different ways to provide strings (you can quote with either @'@ or @"@; the character used to quote has to be escaped with @\@ if used inside the string).
The simple (standard) way @"text"@ has the following escape rules:
* @"\n"@ is a newline, @"\r"@ a carriage return, @"\t"@ a tab stop
* @"\\"@ is one @\@, @"\""@ is one double quote @"@ and @"\'"@ a single quote @'@
* escaping single/double quote is optional if the symbol is not used to terminate the string, i.e. @'\"'@ = @'"'@ and @"\'"@ = @"'"@
* @"\xNN"@: NN must be hexadecimal characters, and the string is replaced with the decoded 8-bit value as a single byte
* All other @\@ occurences are *not* removed from the string.
This way is the preferred one for regular expressions; only to actually match a @\@ you have to do additional escaping (@"\\\\"@; @"\x5C"@ = @"\\"@ is not working), and @\\@ is usually not doing what you wanted (matching a digit: @"\\d"@ = @"\d"@). All other escape rules are compatible with what pcre is doing.
The second way is to place an @e@ before the string like this: @e"text"@. It has the same rules like the normal string, but does not allow unknown escape sequences (the last rule above).
To match a digit with pcre this way you'd have to write @e"\\d"@ instead of @"\d"@.