CODEWITHSUNDEEP
javaregex
Ruan Mendes
Ruan Mendes

Reputation: 92274

Forward slash in Java Regex

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

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

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.

Upvotes: 60

Views: 159210

Answers (3)

James Oravec
James Oravec

Reputation: 20391

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.

Upvotes: 2

coder91
coder91

Reputation: 51

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.

Upvotes: -1

ruakh
ruakh

Reputation: 183280

The problem is actually that you need to double-escape backslashes in the replacement string. You see, "\\/" (as I'm sure you know) means the replacement string is \/, and (as you probably don't know) the replacement string \/ actually just inserts /, because Java is weird, and gives \ a special meaning in the replacement string. (It's supposedly so that \$ will be a literal dollar sign, but I think the real reason is that they wanted to mess with people. Other languages don't do it this way.) So you have to write either:

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

or:

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

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

Upvotes: 92

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