How do I find the characters common to two strings in Java using single replaceAll?

Question

So suppose I have:

String s = \"1479K\";
String t = \"459LP\";

and I want to return

String commonChars = \"49\";
         


        

Answer 1

The accepted answer:

String commonChars = s.replaceAll("[^"+t+"]","");

has a bug!!!

What if the string t has a regex meta-character? In that case the replaceAll fails.

See this program as an example where the string t has ] in it and ] is a regex meta-character which marks the end of the character class. Clearly the program does not produce the expected output.

Why?

Consider:

String s = "1479K";
String t = "459LP]";

Now the regex will become(just substitute t):

String commonChars = s.replaceAll("[^459LP]]","");

Which says replace any character other than 4,5,9,L,P followed by a ] with nothing. Which is clearly not what you want.

To fix these you need to escape the ] in t. You can do it manually as:

String t = "459LP\\]";

and the regex works fine.

This is a common problem when using regex, so the java.util.regex.Pattern class provides a static method named quote which can be used to do exactly this: quote the regex-metacharacters so that they are treated literally.

So before using t in replaceAll you quote it as:

t = Pattern.quote(t);

Program using quote method works as expected.