A possible bug about WordCharacter on CJK characters

This is a pretty ugly hack, but maybe it will inspire you to something better.

leastCJK = ToCharacterCode["⺀"][[1]];
StringMatchQ[#, _?(ToCharacterCode[#][[1]] < leastCJK &)] & /@ 
  {"a", "1", ".", " ", "中", "あ"}

{True, True, True, True, False, False}

Note: "⺀" is unicode character U+2E80, CJK RADICAL REPEAT

This hack can be used with Repeat to ignore CJK strings

StringCases[
  "the quick fox...本当に日本語を勉強していますか...jumped", 
  Repeated[_?(ToCharacterCode[#][[1]] < leastCJK &)], 
  Overlaps -> False]

{"the quick fox...", "...jumped"}

Under the "Implementation Details" section of the official tutorial "Working with String Patterns" we read:

Because PCRE currently does not support preset character classes with characters beyond character code 255, the word and letter character classes (such as WordCharacter and LetterCharacter) only include character codes in the Unicode range 0–255. Thus LetterCharacter and _?LetterQ do not give equivalent results beyond character code 255.

The above-cited Documentation section wasn't updated since Mathematica version 5.2 (July-2005), and shouldn't be correct anymore because starting from PCRE version 8.32 (30-November-2012) the latter supports Unicode:

PCRE_UCP

This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes. By default, only ASCII characters are recognized, but if PCRE_UCP is set, Unicode properties are used instead to classify characters. More details are given in the section on generic character types in the pcrepattern page. If you set PCRE_UCP, matching one of the items it affects takes much longer. The option is available only if PCRE has been compiled with Unicode property support.

<...>

In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff) can be included in a class as a literal string of data units, or by using the \x{ escaping mechanism.

Starting from Mathematica version 10 the two lines of code in the question produce identical results but in general these tests are different, and (as of Mathematica version 11.1.0) the results aren't the same for 705 characters from the Unicode table as one can see from the following:

unicode = FromCharacterCode /@ Range[0, 65276];
test1 = StringMatchQ[#, WordCharacter] & /@ unicode;
test2 = LetterQ[#] || DigitQ[#] & /@ unicode;
Pick[unicode, MapThread[Unequal, {test1, test2}]]