Straight forward slash in Java Regex

Question

I can not understand why the following code is not working properly

"Hello/You/There".replaceAll("/", "\\/"); 

• Expected Result: Hello\/You\/There
• Actual output: Hello/You/There

Do I need to hide slashes? I didn’t think so, but I also tried the following against my will ... did not work

 "Hello/You/There".replaceAll("\\/", "\\/"); 

In the end, I realized that I do not need a regular expression, and I can just use the following, which does not create a regular expression

 "Hello/You/There".replace("/", "\\/"); 

However, I still would like to understand why my first example does not work.

Answer 1

The problem is that you need to double-reset the backslash in the replacement string. You see, "\\/" (as I'm sure you know) means that the replacement string is \/ , and (as you probably don't know), the replacement string \/ actually just inserts / , because Java is weird, and gives \ special value in the replacement string. (It is assumed that \$ will be a literal dollar sign, but I think the real reason is that they want to communicate with people. Other languages ​​do not do it this way.) So you need to write either:

 "Hello/You/There".replaceAll("/", "\\\\/"); 

or

 "Hello/You/There".replaceAll("/", Matcher.quoteReplacement("\\/")); 

(Using java.util.regex.Matcher.quoteReplacement(String) .)

Answer 2

Double escaping is required when presented as a string.

Whenever I create a new regular expression, I do a bunch of tests with online tools, for example: http://www.regexplanet.com/advanced/java/index.html

This website allows you to enter a regular expression that it will put out in a string for you, and you can test it on different inputs.

Answer 3

In fact, there is a reason why all these problems are mixed up. A little more digging is done deeper in this topic and it may be useful to understand the reason why "\\" behaves this way.