String.split() at a meta character +

Question

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);

'+' is a meta char in a regex, use "\\+" in the regex will do. — zw324, Apr 25 '13 at 14:36
@TrigonaMinima I've tried that but then I have a NullPointerEx when applied to this: trim(split(wbana0,'\\\\*')[1]) as name — DiegoMG, Aug 14 '23 at 16:13

Reimeus · Accepted Answer · 2014-10-23T15:17:35.983

58

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("+", " +");

edited Oct 23 '14 at 15:17

answered Apr 25 '13 at 14:36

Reimeus

158,255
15
216
276

Thanks, worked perfectly. I also tried rightside.replace("+", " +") with success but i'm not sure why this worked. – apache Apr 25 '13 at 14:47
`rightside.replace("+", " +")` worked as `String.replace` does not use a regular expression in its target (1st) argument..... – Reimeus Apr 28 '15 at 18:49

score 2 · Answer 2 · answered May 26 '14 at 06:48

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("\\+", " +");

answered May 26 '14 at 06:48

Aniket Thakur

66,731
38
279
289

score 1 · Answer 3 · answered Apr 25 '13 at 14:36

1

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

answered Apr 25 '13 at 14:36

Subhrajyoti Majumder

40,646
13
77
103

score -1 · Answer 4 · answered Feb 03 '21 at 16:30

The reason behind this is - There are reserved characters for regex. So when you split them using the java split() method, You will have to use them with escape. FOr example you want to split by + or * or dot(.) then you will have to do it as split("\+") or split("\*") or split("\.") according to your need.

The reason behind my long explanation on regex is -

YOU MAY FACE IT in OTHER PLACES TOO.

For example the same issue will occur if you use replace or replaceAll methods of java Because they are also working based on regex.

String.split() at a meta character +

4 Answers4

Linked