Full documentation for regex syntax?

Can anyone direct me to the full documentation for the regex syntax? e.g. as provided by the engine Mp3tag uses.

I've seen http://help.mp3tag.de/options_format.html#regexp but it omits e.g. mode modifiers, such as:

Filter expressions

Thanks.

Search the internet for "Boost.Regex".

DD.20140121.1752.CET

Thanks D. Has there been an official statement that Mp3tag regex syntax is the same as Boost.Regex's?

FTR, this error

suggests the syntax is Perl.

So if the engine is Boost, the regexp syntax documentation is this.