String.split() at a meta character +

问题:

I'm making a simple program that will deal with equations from a String input of the equation When I run it, however, I get an exception because of trying to replace the " +" with a " +" so i can split the string at the spaces. How should I go about using

the string replaceAll method to replace these special characters? Below is my code

Exception in thread "main" java.util.regex.PatternSyntaxException: Dangling meta character '+' near index 0 + ^

 public static void parse(String x){
       String z = "x^2+2=2x-1";

       String[] lrside =  z.split("=",4);
       System.out.println("Left side: " + lrside[0] + " / Right Side: " + lrside[1]);
       String rightside = lrside[0];
       String leftside = lrside[1];

       rightside.replaceAll("-", " -");
       rightside.replaceAll("+", " +");
       leftside.replaceAll("-", " -"); leftside.replaceAll("+", " +");
       List<String> rightt = Arrays.asList(rightside.split(" "));
       List<String> leftt = Arrays.asList(leftside.split(" "));

       System.out.println(leftt);
       System.out.println(rightt);

回答1:

replaceAll accepts a regular expression as its first argument.

+ is a special character which denotes a quantifier meaning one or more occurrences. Therefore it should be escaped to specify the literal character +:

rightside = rightside.replaceAll("\\+", " +");

(Strings are immutable so it is necessary to assign the variable to the result of replaceAll);

An alternative to this is to use a character class which removes the metacharacter status:

rightside = rightside.replaceAll("[+]", " +");

The simplest solution though would be to use the replace method which uses non-regex String literals:

rightside = rightside.replace("+", " +"); 

回答2:

I had similar problem with regex = "?". It happens for all special characters that have some meaning in a regex. So you need to have "\\" as a prefix to your regex.

rightside = rightside.replaceAll("\\+", " +");

回答3:

String#replaceAll expects regex as input, and + is not proper pattern, \\+ would be pattern. rightside.replaceAll("\\+", " +");