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It is much faster to split with a UnicodeSet than with a regex. Here are some figures:

String.split Timing /[,;]/ 767ns
Pattern.split Timing /[,;]/ 482ns
UnicodeSet.split Timing /[,;]/ 282ns

String.split Timing /\p{P}/ 1,114ns
Pattern.split Timing /\p{P}/ 847ns
UnicodeSet.split Timing /\p{P}/ 281ns

10,000,000 iterations, with warmup.

This discusses a draft for an API. Feedback is welcome.

The idea is to have a Splitter base class, and two concrete implementations.
  • UnicodeSetSplitter
  • RegexSplitter
  • use a builder, or have freeze(), or leave mutable?
  • for UnicodeSetSplitter, we could freeze the input, clone and freeze, or leave alone (faster if client supplies frozen set).
The return values (depending on language and which method you call) would be:
  • Java: List<String> or String[] (input could be a CharSequence anyway)
    • Or an Iterator<String> which would expose the most natural implementation
    • A Java Iterator and a C++ StringEnumeration could split the input string on the fly. If the caller stops iterating, then no further boundary search and object creation is wasted. Only when StringEnumeration::count() is called would it have to split and cache to the end.
  • C++: StringEnumeration or array+count (input parameter: array of UnicodeString pointers to be filled in)

Examples for illustrating settings. 
pattern = [,;]
text = "ab ; cd ,"
  1. Normal
    • ["ab ", " cd ", ""]
  2. Maximum number of resulting items:
    • maximumCount=1 results in ["ab ", " cd ,"]
    • Question: return the rest of the string in an extra element?
  3. Should each string be trimmed or not (supplying a UnicodeSet for the trimming)
    • trim=[:space:] results in ["ab", "cd", ""]
  4. Omit trailing strings (Perl/Java compat mode)
    • omitTrailing=true results in ["ab ", " cd "]
  5. Include separators
    1. includeSeparators=true results in ["ab ", ";", " cd ", ",", ""]
    2. odd elements are the separators found.
The longest match is used. So with the UnicodeSet [< = > {<=} {>=}], then "a<=b" results in ["a", "b"]

Possible future settings
(but would need use cases).

UnicodeSet only (can already do with Regex)
  1. match shortest : "a<=b" => ["a", "", "b"] // using above example
  2. match "SIMPLE" sequence of separators: [,;] with "a;,;b" results in ["a", "b"]
  3. match "CONTAINED" sequence of separators: [{ab}, {abc}, {cd}] with "xabcdy" results in ["x", "y"]