Backslash problem with String.replaceAll

Question

I'm trying to convert "\something\" into "\\something\\" using replaceAll, but I keep getting all kinds of errors. I thought this was the solution:

theString.replaceAll("\\", "\\\\");

But this gives: Exception in thread "main" java.util.regex.PatternSyntaxException: Unexpected internal error near index 1

Answer 1

You'll need to escape the (escaped) backslash in the first argument as it is a regular expression. Replacement (2nd argument - see Matcher#replaceAll(String)) also has it's special meaning of backslashes, so you'll have to replace those to:

theString.replaceAll("\\\\", "\\\\\\\\");

Answer 2

Yes... by the time the regex compiler sees the pattern you've given it, it sees only a single backslash (since Java's lexer has turned the double backwhack into a single one). You need to replace "\\\\" with "\\\\", believe it or not! Java really needs a good raw string syntax.

Answer 3

To avoid this sort of trouble, you can use replace (which takes a plain string) instead of replaceAll (which takes a regular expression). You will still need to escape backslashes, but not in the wild ways required with regular expressions.

Answer 4

Something says me that you're trying to use Windows-like file system paths. If this is true, then it may be useful to know that you can also just use the forward slash as path separator in windows.

As to your actual problem: the \ is an escape character in both the String and in regex. You need to re-escape it as well:

string.replaceAll("\\\\", "\\\\\\\\");

But you don't necessarily need regex for this, simply because you want an exact character-by-character replace and you don't need patterns here. So String#replace() would suffice:

string.replace("\\", "\\\\");