Java用特殊字符分割

Question

I have below code that doing a split for string using <div>\\\\$\\\\$PZ\\\\$\\\\$</div> and it's not working using the special characters.

public class HelloWorld{

     public class HelloWorld{

     public static void main(String []args){
          String str = "test<div>\\$\\$PZ\\$\\$</div>test"; 
        String[] arrOfStr = str.split("<div>\\$\\$PZ\\$\\$</div>", 2); 
        for (String a : arrOfStr) 
            System.out.println(a);
     }
}

the output os test<div>\\$\\$PZ\\$\\$</div>test

it works when I remove the special characters

Can you please help.

Answer 1

As you already know, the parameter to split(...) is a regular expression , so some characters have special meaning. If you want the parameter to be treated literally , ie not as a regex, call the Pattern.quote(String s) method.

Example

String str = "test<div>\\$\\$PZ\\$\\$</div>test";
String[] arrOfStr = str.split(Pattern.quote("<div>\\$\\$PZ\\$\\$</div>"), 2);
for (String a : arrOfStr)
    System.out.println(a);

Output

test
test

The quote() method simply surrounds the literal text with the regex \\Q...\\E quotation pattern ¹ , eg your <div>\\$\\$PZ\\$\\$</div> text becomes:

\Q<div>\$\$PZ\$\$</div>\E

For fixed text you could just do that yourself, ie the following 3 versions all create the same regex to split on:

str.split(Pattern.quote("<div>\\$\\$PZ\\$\\$</div>"), 2)

str.split("\\Q<div>\\$\\$PZ\\$\\$</div>\\E", 2)

str.split("<div>\\\\\\$\\\\\\$PZ\\\\\\$\\\\\\$</div>", 2)

To me, the 3rd one, using \\ to escape, is the least readable/desirable version.

If there is a lot of special characters to escape, using \\Q...\\E is easier than \\ -escaping all the special characters separately, but very few people use it, so it's fairly unknown to most.

The quote() method is especially useful when you need to treat dynamic text literally, eg when the text to split on is configurable by the user.

^{1) quote() will correctly handle literal text containing \\E .}

Answer 2

This:

String str = "test<div>\\$\\$PZ\\$\\$</div>test";
String[] arrOfStr = str.split("<div>\\\\\\$\\\\\\$PZ\\\\\\$\\\\\\$</div>", 2);
for (String a : arrOfStr) {
    System.out.println(a);
}

prints:

test
test

EDIT : Why do we need all those backslashes? It's because of how we need to handle String literals representing regex expressions. This page describes the reason with examples. The essence is this:

For a backslash \\ ...

...the pattern to match that would be \\\\ ... (to escape the escape)

... but the string literal to create that pattern would have to have one backslash to escape each of the two backslashes: \\\\\\\\ .

Add to that the original need to also escape the $ , that gives us our 6 backslashes in the string representation.