不匹配时Java replaceAll方法返回null

Question

I have some existing Java code

Pattern regex = Pattern.compile(regexStr, Pattern.CASE_INSENSITIVE);        
Matcher m = regex.matcher(value);
String replace= m.replaceAll(regexReplaceStr);

This is hard-coded in my project.I can't change it. I can only set values regexStr and regexReplaceStr by configuration. What I need to do is

• If there is a match, return the original string
• If there is no match, return an empty string

What should my strings regexStr and regexReplaceStr be in this case?

So for example, I have

regexStr = "^[^_]+_(.*)$"
regexReplace = "$0"

So now if the input string is AA_BBB , the value returned is AA_BBB . Now, if the input string is AABBB , I want it to return an empty string . How can this be done?

Answer 1

First of all, the code is unclear. There is no reference to 'value'...

Matcher m = regex.matcher(value);
                           ^ ^

And there is also no context about this piece of code.
Is it used as a method? A user input?

You could add this after your code:

if (replace == null) {
    replace = "";
}

It would then replace null by an empty String.

However I did the same code as you but it never returned null...