[java] Forward slash in Java Regex

I can't figure out why the following code doesn't behave as expected

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

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

Do I need to escape forward slashes? I didn't think so but I also tried the following against my will ... didn't work

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

In the end I realized I don't need a regular expression and I can just use the following, which doesn't create a regular expression

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

However, I'd still like to understand why my first example doesn't work.

There is actually a reason behind why all these are messed up. A little more digging deeper is done in this thread and might be helpful to understand the reason why "\\" behaves like this.

Double escaping is required when presented as a string.

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

That website allows you to enter the regular expression, which it'll escape into a string for you, and you can then test it against different inputs.