+1 to this request. For me, I'd leave case sensitivity enabled by default, which usually the case for PCRE, see regex flags here https://regex101.com/ (click the '/g'). If possible, I'd incorporate the PCRE flags as the fourth optional parameter.
For Wolfgang Amadeus Mozart > Mozart, Wolfgang Amadeus:
$regexp as proposed by stevehero:
$regexp(str,expr,replace,optional flag)
$regexp(%Composer%,'(^.*)(?:\s)(\w+$)',$2', '$1,i) [note: the case insensitive "i" flag is not really needed here]
$regexp PCRE-style as seen in https://regex101.com without the fourth optional parameter, as it's already incorporated. Pro's: less need to segregate regex search and replace, two parameters instead of potentially four; easily debugged with help from others using https://regex101.com/. Con: might be a bit more confusing to users who are not used to unix-style regex search and replace
$regexp(%field%,'/search/replace/optional flags')
$regexp(%Composer%,'/(^.*)(?:\s)(\w+$)/$2, $1/i') [note: the case insensitive "i" flag is not really needed here]
$regexp(%Composer%,'/(^.*)(?:\s)(\w+$)/$2, $1/')
Currently in fb2k:
$right($ifgreater($meta_num(composer),1,$meta_sep(composer,', ',' & '),%composer%),$sub($len($ifgreater($meta_num(composer),1,$meta_sep(composer,', ',' & '),%composer%)),$strrchr($ifgreater($meta_num(composer),1,$meta_sep(composer,', ',' & '),%composer%),' ')))', '$left($ifgreater($meta_num(composer),1,$meta_sep(composer,', ',' & '),%composer%),$sub($strrchr($ifgreater($meta_num(composer),1,$meta_sep(composer,', ',' & '),%composer%),' '),1))
I know I'll go with the regex options one 100% of the time.