Java String.replaceAll with different match & replace criteria

Question

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.

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
– Bohemian ♦, Commented Jul 9, 2013 at 23:29

Bernhard Barker · Accepted Answer · 2013-07-09 22:54:01Z

4

You can use groups for this:

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

$1 refers to the first thing in brackets.

Java regex reference.

answered Jul 9, 2013 at 22:54

Bernhard Barker

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2 Comments

MadDogMcNamara Over a year ago

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

Bernhard Barker Over a year ago

@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).

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