Replace a single character within a match via 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.