Late reply cause finding time is hard. That functions page is a bit sparse when coming into the code world cold, was using it with my experimenting before.
It's hard to know exactly what you're trying to do without seeing other examples of your tags
My bad, should of given more context, at the same time didn't want to just dump my problem fully onto the community, difficult balance =]
Full outline of my tagging wish: To display the <Artist> when it differs from <Album Artist> (eg Various Artists, Soundtrack) but when the difference is minor like a featured artist or a remix to only display that difference, that way it's more likely visible before truncation.

eg: "Mass [EOB [ft: Laura Marling]]"
==: "Mass [ft: Laura Marling]"
Since I consistently use square brackets for my featured artists or remixes this makes it a good split point like you helped me initially, but I needed to add the ability to display the full <Artist> in cases where they differed for whatever reason from the <Album Artist> so a good example would be VA or Soundtracks* (which I use as <Album Artist> to group Soundtracks).

This is the same functionality as MB defaults, I just needed to add this back in without causing duplicates where a square bracket might be used.

eg: "Piano Black [Seatbelts [rmx: Ian O'Brien]] [rmx: Ian O'Brien]"
==: "Piano Black [Seatbelts [rmx: Ian O'Brien]]"
I'm only doing comparative jobs on <Artist> and <Album Artist> boom boom, I'm fussy about my labelling but that amount of precision is beyond me (impressive but daunting, meant as a compliment ^^)
These screenshots show how I
do want the titles to display, I got it to work but it feels messy.
<Title>$If($Contains(<Artist>,<Album Artist>),$If($Contains(<Artist>,[),{Contrast: 35}" ["$RxSplit(<Artist>,"\[",2),),{Contrast: 35}" ["<Artist>"]")
If <artist> contains <album artist>
(T) if <artist> has a "["
(T) display text from after that "[",
(F) display nothing from <artist>
(F) display <artist>
Thanks again, if I get a reply pointing out something to neaten it up I will, might post a tips post about this with area's denoted where it might apply to their tagging scheme and also a sorting virtual tag with mention of users like yourselves who basically did the work / heavily guided me if you guys aren't opposed?