regex - How to split String with some separator but without removing that separator in Java?



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5 Answers: 


This works because split actually takes a regular expression. What you're actually seeing is a "zero-width positive lookahead".

I would love to explain more but my daughter wants to play tea party. :)

Edit: Back!

To explain this, I will first show you a different split operation:


This splits your string on every zero-length string. There is a zero-length string between every character. Therefore, the result is:

["", "R", "a", "m", "-", "s", "i", "t", "a", "-", "l", "a", "x", "m", "a", "n"]

Now, I modify my regular expression ("") to only match zero-length strings if they are followed by a dash.

["Ram", "-sita", "-laxman"]

In that example, the ?= means "lookahead". More specifically, it mean "positive lookahead". Why the "positive"? Because you can also have negative lookahead (?!) which will split on every zero-length string that is not followed by a dash:

["", "R", "a", "m-", "s", "i", "t", "a-", "l", "a", "x", "m", "a", "n"]

You can also have positive lookbehind (?<=) which will split on every zero-length string that is preceded by a dash:

["Ram-", "sita-", "laxman"]

Finally, you can also have negative lookbehind (?<!) which will split on every zero-length string that is not preceded by a dash:

["", "R", "a", "m", "-s", "i", "t", "a", "-l", "a", "x", "m", "a", "n"]

These four expressions are collectively known as the lookaround expressions.

Bonus: Putting them together

I just wanted to show an example I encountered recently that combines two of the lookaround expressions. Suppose you wish to split a CapitalCase identifier up into its tokens:

"MyAwesomeClass" => ["My", "Awesome", "Class"]

You can accomplish this using this regular expression:


This splits on every zero-length string that is preceded by a lower case letter ((?<=[a-z])) and followed by an upper case letter ((?=[A-Z])).

This technique also works with camelCase identifiers.


It's a bit dodgy, but you could introduce a dummy separator using a replace function. I don't know the Java methods, but in C# it could be something like:

string1.Replace("-", "#-").Split("#");

Of course, you'd need to pick a dummy separator that's guaranteed not to be anywhere else in the string.


A way to do this is to split your string, then add your separator at the beginning of each extracted string except the first one.

String[] splitstrings = string1.split(seperator);
for(int i=1; i<splitstring.length;i++)
   splitstring[i] = seperator + splitstring[i];

that is the code fitting to LadaRaider's answer.


Adam hit the nail on the head! I used his answer to figure out how to insert filename text from the file dialog browser into a rich text box. The problem I ran into was when I was adding a new line at the "\" in the file string. The string.split command was splitting at the \ and deleting it. After using a mixture of Adam's code I was able to create a new line after each \ in the file name.

Here is the code I used:

OpenFileDialog fd = new OpenFileDialog();
        fd.Multiselect = true;

        foreach (string filename in fd.FileNames)
            string currentfiles = uxFiles.Text;
            string value = "\r\n" + filename;

     //This line allows the Regex command to split after each \ in the filename. 

            string[] lines = Regex.Split(value, @"(?<=\\)");

            foreach (string line in lines)
                uxFiles.Text = uxFiles.Text + line + "\r\n";