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I have this code:

String polynomial = "2x^2-4x+16";
String input = polynomial.replaceAll("[0-9a-zA-Z][-]", "+-");

The problem is I don't want to actually replace the [0-9a-zA-Z] char.

Previously, I had used polynomial.replace("-","+-"); but that gave incorrect output with negative powers.

The new criteria [0-9a-zA-Z][-] solves the negative power issue; however it replaces a char when I only need to insert the + before the - without deleting that char.

How can I replace this pattern using the char removed like: 

polynomial.replaceAll("[0-9a-zA-Z][-]", c+"+-");

where 'c' represents that [0-9a-zA-Z] char.

share|improve this question
    
What is it you're trying to do? Just give example input and output. All the info about your attempts is meaningless without context. –  Bohemian♦ Jul 9 '13 at 23:29

1 Answer 1

up vote 1 down vote accepted

You can use groups for this:

polynomial.replaceAll("([0-9a-zA-Z])[-]", "$1+-");

$1 refers to the first thing in brackets.

Java regex reference.

share|improve this answer
    
Exactly what I needed, thanks. –  user1094607 Jul 9 '13 at 22:55
    
docs.oracle.com/javase/tutorial/essential/regex/groups.html : There it says that backreferences are made by \1, \2 ... \n, not $1. I don't know that $1, ... won't work, but thought I'd mention it –  MadDogMcNamara Jul 9 '13 at 22:55
    
@MadDogMcNamara $1 does work. –  user1094607 Jul 9 '13 at 22:57
1  
@MadDogMcNamara \1 is for group references from inside the regular expression while $1 is for group references in replace... (following a few links from String.replaceAll gets you here). –  Dukeling Jul 9 '13 at 23:02
    
Ah thank you. I was clearly confused. –  MadDogMcNamara Jul 9 '13 at 23:37

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