Replace a single character within a match via Regex - regex

Question

I have a simple pattern that I am trying to do a find and replace on. It needs to replace all dashes with periods when they are surrounded by numbers.
Replace the period in these:
3-54
32-11
111-4523mhz
Like So:
3.54
32.11
111.4523mhz
However, I do not want to replace the dash inside anything like these:
Example-One
A-Test
I have tried using the following:
preg_replace('/[0-9](-)[0-9]/', '.', $string);
However, this will replace the entire match instead of just the middle. How do you only replace a portion of the match?

Answer 1

preg_replace('/([0-9])-([0-9])/', '$1.$2', $string);
Should do the trick :)
Edit: some more explanation:
By using ( and ) in a regular expression, you create a group. That group can be used in the replacement. $1 get replaced with the first matched group, $2 gets replaced with the second matched group and so on.
That means that if you would (just for example) change '$1.$2' to '$2.$1', the operation would swap the two numbers.
That isn't useful behavior in your case, but perhaps it helps you understand the principle better.

Answer 2

Depending on the regex implementation you're using, you can use non-capturing groups:
preg_replace('/(?<=[0-9])-(?=[0-9])/', '.', $string);

Answer 3

You can use back-references to retain the parts of the match you want to keep:
preg_replace('/([0-9])-([0-9])/', '$1.$2', $string);

Answer 4

Below are couple of ways of achieving the same.
import java.util.Arrays;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
/**
* Created by Rituraj_Jain on 7/14/2017.
*/
public class Main {
private static final Pattern pattern = Pattern.compile("(\\S)-(\\S)");
private static final String input = "Test-TestBad-ManRituRaj-Jain";
public static void main(String[] args) {
Matcher matcher = pattern.matcher(input);
String s = matcher.replaceAll("$1 $2");
System.out.println(s);
System.out.println(input.replaceAll(pattern.pattern(), "$1 $2"));
}
public static void main1(String[] args) {
Matcher matcher = pattern.matcher(input);
StringBuffer finalString = new StringBuffer();
while(matcher.find()){
String replace = matcher.group().replace("-", " ");
matcher.appendReplacement(finalString, replace);
}
matcher.appendTail(finalString);
System.out.println(input);
System.out.println(finalString.toString());
}
}