Regex to remove spaces between numbers only

How about:

message = "Hello 111 222 333 World!".replaceAll("(\\d)\\s(\\d)", "$1$2");

Gives:

"Hello 111222333 World!"

You can use this lookaround based regex:

 String repl = "Hello 111 222 333 World!".replaceAll("(?<=\\d) +(?=\\d)", "");
 //=> Hello 111222333 World!

This regex "(?<=\\d) +(?=\\d)" makes sure to match space that are preceded and followed by a digit.

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