Escaping double-slashes with regular expressions in Java

Question

I have this unit test:

public void testDeEscapeResponse() {
    final String[] inputs = new String[] {"peque\\\\u0f1o", "peque\\u0f1o"};
    final String[] expected = new String[] {"peque\\u0f1o", "peque\\u0f1o"};
    for (int i = 0; i < inputs.length; i++) {
        final String input = inputs[i];
        final String actual = QTIResultParser.deEscapeResponse(input);
        Assert.assertEquals(
            "deEscapeResponse did not work correctly", expected[i], actual);
    }
}

I have this method:

static String deEscapeResponse(String str) {
    return str.replaceAll("\\\\", "\\");
}

The unit test is failing with this error:

java.lang.StringIndexOutOfBoundsException: String index out of range: 1
    at java.lang.String.charAt(String.java:686)
    at java.util.regex.Matcher.appendReplacement(Matcher.java:703)
    at java.util.regex.Matcher.replaceAll(Matcher.java:813)
    at java.lang.String.replaceAll(String.java:2189)
    at com.acme.MyClass.deEscapeResponse
    at com.acme.MyClassTest.testDeEscapeResponse

Why?

Breaststroke

Answer 1

Use String.replace which does a literal replacement instead of String.replaceAll which uses regular expressions.

Example:

"peque\\\\u0f1o".replace("\\\\", "\\")    //  gives  peque\u0f1o

---

String.replaceAll takes a regular expression thus \\\\ is interpreted as the expression \\ which in turn matches a single \. (The replacement string also has special treatment for \ so there's an error there too.)

To make String.replaceAll work as you expect here, you would need to do

"peque\\\\u0f1o".replaceAll("\\\\\\\\", "\\\\")

Staff

You know, I thought about using replace, but I thought it was exactly the same as replaceAll, except that it only replaced the first instance. Thanks!! – Breaststroke

Ah, I can see why you thought so when there is a replaceAll method :-) – Staff

...though there is actually a String.replaceFirst method for that :-) – Staff

Answer 2

I think the problem is that you're using replaceAll() instead of replace(). replaceAll expects a regular expression in the first field and you're just trying to string match.

Nauplius

Answer 3

See javadoc for Matcher:

Note that backslashes (\) and dollar signs ($) in the replacement string may cause the results to be different than if it were being treated as a literal replacement string. Dollar signs may be treated as references to captured subsequences as described above, and backslashes are used to escape literal characters in the replacement string.

Thus with replaceAll you cannot replace anything with a backslash. Thus a really crazy workaround for your case would be str.replaceAll("\\\\(\\\\)", "$1")

Enaenable