Describe the type of regex used
The documentation (help/C/xinclude-filter-vfolder-conditions.xml) describes the Regex Match condition as:
(For programmers only) If you know your way around a <link href="https://en.wikipedia.org/wiki/Regular_expression">regex</link>, or regular expression, this option allows you to search for complex patterns of letters, so that you can find, for example, all words that start with "a" and end with "m", and are between six and fifteen letters long, or all messages that declare a particular header twice. For information about how to use regular expressions, check <link href="man:grep">the man page for the <cmd>grep</cmd></link> command.
It points to grep, but doesn't really describe the kind of regex it uses. It actually is an Extended Regular Expression, as it is using regexec(3) in eds camel-search-private.c from a regex compiled at camel_search_build_match_regex, which unconditionally sets the REG_EXTENDED
flag.
I'm also not keen on its characterization as "For programmers only", either. It can probably be improved as a whole.