Matching (e.g.) a Unicode letter with Java regexps

Question

There are many questions and answers here on StackOverflow that assume a \"letter\" can be matched in a regexp by [a-zA-Z]. However with Unicode there are many

Answer 1

Here you have a very nice explanation:

http://www.regular-expressions.info/unicode.html

Some hints:

"Java and .NET unfortunately do not support \X (yet). Use \P{M}\p{M}* as a substitute. To match any number of graphemes, use (?:\P{M}\p{M}*)+ instead of \X+."

"In Java, the regex token \uFFFF only matches the specified code point, even when you turned on canonical equivalence. However, the same syntax \uFFFF is also used to insert Unicode characters into literal strings in the Java source code. Pattern.compile("\u00E0") will match both the single-code-point and double-code-point encodings of à, while Pattern.compile("\\u00E0") matches only the single-code-point version. Remember that when writing a regex as a Java string literal, backslashes must be escaped. The former Java code compiles the regex à, while the latter compiles \u00E0. Depending on what you're doing, the difference may be significant."