Why does replaceAll fail with "illegal group reference"?

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最佳答案

Use "\\$\\$" in the second parameter:

String s="  $$";
s=s.replaceAll("\\s+\\$\\$","\\$\\$");
//or
//s=s.replaceAll("\\s+\\Q$$\\E","\\$\\$");

The $ is group symbol in regex's replacement parameter

So you need to escape it

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The problem here is not the regular expression, but the replacement:

$ is used to refer to () matching groups. So you need to escape it as well with a backslash (and a second backslash to make the java compiler happy):

String s="  $$";
s = s.replaceAll("\\s+\\$\\$", "\\$\\$");

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$ has special meaning in the replacement string as well as in the regex, so you have to escape it there, too:

s=s.replaceAll("\\s+\\$\\$", "\\$\\$");

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String s="$$";
s=s.replaceAll("\\s+\\$\\$","$$");

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From String#replaceAll javadoc:

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; see Matcher.replaceAll. Use Matcher.quoteReplacement(java.lang.String) to suppress the special meaning of these characters, if desired.

So escaping of an arbitrary replacement string can be done using Matcher#quoteReplacement:

String s = "  $$";
s = s.replaceAll("\\s+\\$\\$", Matcher.quoteReplacement("$$"));

Also escaping of the pattern can be done with Pattern#quote

String s = "  $$";
s = s.replaceAll("\\s+" + Pattern.quote("$$"), Matcher.quoteReplacement("$$"));

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I had the same problem, so I end up implementing replace all with split.
It solved the exception for me

public static String replaceAll(String source, String key, String value){
String[] split = source.split(Pattern.quote(key));
StringBuilder builder = new StringBuilder();
builder.append(split[0]);
for (int i = 1; i < split.length; i++) {
builder.append(value);
builder.append(split[i]);
}
while (source.endsWith(key)) {
builder.append(value);
source = source.substring(0, source.length() - key.length());
}
return builder.toString();
}

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This is the right way. Replace the literar $ by escaped \\$ str.replaceAll("\\$", "\\\\\\$")

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import java.util.regex.Pattern;
import java.util.regex.Matcher;
public class HelloWorld{


public static void main(String []args){
String msg = "I have %s in my string";


msg = msg.replaceFirst(Pattern.quote("%s"), Matcher.quoteReplacement("$"));
System.out.println(msg);
}

}