import java.io.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
 
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		String regex = "(\\w+\\s+\\+\\s+\\w+\\s+-->\\s+\\w+|\\w+\\s+-->\\s+\\w+\\s+\\+\\s+\\w+)[\\s\\n\\r]+(\\w+\\s+\\+\\s+\\w+\\s+-->\\s+\\w+|\\w+\\s+-->\\s+\\w+\\s+\\+\\s+\\w+)";
		String string = "a + b --> c\n\n"
			 + "c + d --> e\n"
			 + "The empty line after this is left alone\n\n"
			 + "e + a --> b\n\n"
			 + "a --> b + c";
		String subst = "$1\\n$2";
 
		Pattern pattern = Pattern.compile(regex, Pattern.DOTALL);
		Matcher matcher = pattern.matcher(string);
		String result = matcher.replaceAll(subst);
 
		System.out.println(result);
	}
}

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