Remove empty line from a multi-line string with Java

Use regex (?m)^[ \t]*\r?\n" to remove empty lines:

log.info msg.replaceAll("(?m)^[ \t]*\r?\n", "");

To remain only 1 line use [\\\r\\\n]+:

log.info text.replaceAll("[\\\r\\\n]+", "");

If you want to use the value later, then assign it

text = text.replaceAll("[\\\r\\\n]+", "");

Your current attempt seems very much on the right track, and I think the logic you want is to replace two or more newlines with just a single newline:

output = msg.replaceAll("(\r?\n)(?:\r?\n){1,}", "$1");

The trick here, to capture the actual newline type being used (\n for Linux or \r\n for Windows), is to capture a single \r?\n first, followed then by at least one more newline.