Regex matching Java

Question

I have a problem regex matching an upper case letter possibly followed by a lower case letter. I want to break after any such matches, but I just can't seem to get it to work.

To make it more general - I want to split before and after any matches in regex.

Example string "TeSTString"

Wanted result -> [Te, S, T, St, ring]

I have tried anything I can think of, but I'm getting tricked by look-ahead or behind.

First I tried [AZ][az]? , and that matches perfect, but removes it...

result -> [ring]

after this I did positive look-ahead (?=([AZ][az]?)) giving me something close...

result -> [Te, S, T, String]

and look-behind (<=?([AZ][az]?)) giving nothing at all...

result -> [TeSTString]

even tried reversing the look-behind (<=?([az]?[AZ])) , in a desperate attempt, but this was fairly unsuccessful.

Can anyone give a good pointer in the right direction before I lose my mind?

Answer 1

Here's one convoluted pattern that will match the expected result.

String test = "TeSTStringOne";
System.out.println(
    Arrays.toString(
        //          | preceded by lowercase
        //          |        | followed by uppercase
        //          |        |       | or
        //          |        |       || preceded and followed by uppercase
        //          |        |       ||                  | or
        //          |        |       ||                  || preceded by uc
        //          |        |       ||                  || AND lowercase
        test.split("(?<=[a-z])(?=[A-Z])|(?<=[A-Z])(?=[A-Z])|(?<=[A-Z][a-z])")
    )
);

Output

[Te, S, T, St, ring, On, e]

Note

Replace [az] with \\\\p{Ll} and [AZ] with \\\\p{Lu} to use with accented letters.

Answer 2

Try with:

(?<=[A-Z][a-z])|(?=(?<!^)[A-Z])

DEMO

• (?<=[AZ][az]) = positive lookbehind for upper case followed by lower case,
• (?=(?<!^)[AZ]) - positive lookahead for upper case, if not preceded by beginnig of a line,