Using Java regexes to match a range of Unicode code points _outside_ the BMP: it is possible at all?

Question

While totally unrelated at first, this question made me wonder...

Java's regexes are based on Strings; Strings are sequences (arrays) of chars; and chars are ultimately UTF-16 code units.

The latter means that a single char can match any Unicode code point inside the BMP, ie from U+0000 to U+FFFF.

Outside the BMP however, two chars are required for a single code point (one for the leading surrogate, another for the trailing surrogate); from what I can see, apart from a dedicated grammar engine, I don't see a way for Java regexes (as defined by java.util.regex.Pattern) to define "character classes" for such code points, since there is no String literal for code points outside the BMP.

Notwithstanding that code can be written to produce regexes (well, string literals used as regexes) for such ranges, is there an existing mechanism in Pattern which is not documented and allows to do that?

Answer 1

OK, so, answer to self; data extracted from this question and the associated answer which @DavidWallace pointed to.

It is possible. To paraphrase the answer, in such a character class as:

"[\uD83D\uDE01-\uD83D\uDE4F]"

the Java regex engine will be smart enough to notice that you specify a surrogate pair on both ends of the interval, and "compile" the regex accordingly.

In addition, starting with Java 7, you can also use \x{foo} where foo is the hexadecimal representation of the code point. Not forgetting the quoting necessary in Java string literals, the above can therefore be written:

"[\\x{1F601}-\\x{1F64F}]"